Font Smoothing in Edge

Update, June 2021: See the Microsoft Edge blog post.

Text rendering quality is an amazingly complicated topic, with hardware, settings, fonts, differing rendering engine philosophies, and user preferences all playing key roles. In some cases, however, almost everyone can agree that one rendering is superior to another. Consider, for instance, the text of this Gizmodo article as seen on one user’s computer:

You can use this fancy swipe-view widget to wipe between the renderings of the full paragraph:

Most people think the text for Edge looks awful, with unexpectedly chunky letters and irregular kerning, but the text for IE11 looks pretty good.

Investigation reveals that the problem here is that Edge and Firefox are respecting the system’s font smoothing setting, but IE11 is ignoring it.

Font Smoothing in Windows

Windows has three levels of font smoothing: Off, Basic/Standard, and ClearType. Here’s a quick chart showing the impact of each setting across three browser engines:

Notably, the IE11 rendering is pixel-for-pixel identical regardless of Windows settings– it renders with grayscale subpixel smoothing even when smoothing is off or ClearType is enabled. In contrast, if you zoom into the ClearType examples in Edge 86 and Firefox 80 you can see subpixel smoothing at work, with tiny colored fringes smoothing the edges of the characters.

Examining Smoothing Parameters

Font Smoothing is controlled by four registry values inside HKCU\Control Panel\Desktop. FontSmoothing supports two values {0=Off,2=On} and FontSmoothingType supports values {1=Basic,2=ClearType}. The FontSmoothingGamma parameter controls the darkness of the smoothing and accepts values between 1000 and 2200. You have to zoom in pretty close to see the effect:

The FontSmoothingOrientation flag controls the order of the red, blue, and green pixels in the display; it supports two values {0=BGR, 1=RGB}; ClearType needs this information to understand which subpixels to illuminate when smoothing. RGB is the most common and the default.

Applications that need this information should use the SystemParametersInfo function to retrieve these parameters.

Tuning Parameters

End-users can enable FontSmoothing in the Windows Performance Options (Win+R, then SystemPropertiesPerformance.exe):

To enable ClearType and tune its settings for your displays and settings, run the ClearType Tuner Wizard (Win+R, then CTTune.exe):

The Tuner will walk you through a series of side-by-side text renderings, asking which of them looks best, a bit like an eye doctor determining the parameters for your prescription eyeglasses.

Note: If you use a Windows PC via a remote desktop connection, ensure that the “Font Smoothing” option is checked in the connection properties:

…and that font smoothing wasn’t disabled via policy on the remote server.

Checking Edge Status

You can determine what font smoothing method is presently used in Edge by visiting the URL edge://histograms/Microsoft.Fonts.FontSmoothingMethod

The histogram will show a datapoint for the current state, where

Other Culprits?

Windows settings do not account for all cases of text rendering dissatisfaction.

In some cases, the problem is that a website has selected a font not present on the user’s PC, forcing fallback to an inferior font lacking proper hinting data for smoothing. Within Chromium itself, the browser changes the default fixed width font from Courier New to Consolas if ClearType is enabled, because the latter has better hinting information. Similarly, in Edge 85, we improved font fallback for Chinese to prefer the (ClearType-optimized) Microsoft YaHei and Microsoft JhengHei fonts over legacy fonts.

In other cases, users may simply prefer darker text than ClearType generates, perhaps using a browser extension to achieve their preferences.

In other cases, the user’s hardware might not be optimally configured for font smoothing. For instance, if you run a monitor in Portrait mode, its pixels have a different layout. A device can report its subpixel geometry using a registry key.

If you see a case of poor text rendering across browsers that you cannot explain using the information in this post, please let us know about it!

Update, June 2021: See the Microsoft Edge blog post.


Managing Edge via Policy

The new Microsoft Edge offers a rich set of policies that enable IT administrators to control many aspects of its operation.

You can visit edge://policy/ to see the policies in effect in your current browser:

Clicking on a policy name will take you to the documentation for that policy. The Status column indicates whether the policy is in effect, in Error, or Ignored. A policy is in Error if the policy name is unrecognized or the policy value is malformed. A policy is Ignored if the policy is a Protected Policy and the machine is not Domain Joined or MDM managed. Policies are marked “Protected” if they are especially often abused by malware. For instance, policies controlling the content of the New Tab Page are protected because adware/malware commonly attempted to monetize users by silently changing their search engine and homepage when their “free” apps were installed on a user’s PC. Protected Policies are marked in the Edge documentation with the note:

This policy is available only on Windows instances that are joined to a Microsoft Active Directory domain, Windows 10 Pro or Enterprise instances that enrolled for device management, or macOS instances that are that are managed via MDM or joined to a domain via MCX.

When Edge detects that a device is in a managed state in which Protected Policies are allowed, it will show “Managed by your organization” at the bottom of the … menu

Implementation mechanism

There’s no magic in how policies are implemented: while you should prefer using edge://policy to look at policies to get Edge’s own perspective about what policies are set, you can also view (and set) policies using the Windows Registry:

Careful, this thing is loaded…

You must take great care when configuring policies, as they are deliberately much more powerful than the options exposed to end-users. In particular, it is possible to set policies that will render the browser and the device it runs on vulnerable to attack from malicious websites.

Administrators should take great care when relaxing security restrictions through policy to avoid opening clients up to attack. For instance, avoid using entries like https://* in URLList permission controls– while such a rule may cover all of your Intranet Zone sites, it also includes any malicious site on the Internet using HTTPS.

… but Incomplete

Notably, not all settings in the browser can be controlled via policy. For instance, some of the web platform feature settings inside edge://settings/content can only be enabled/disabled entirely (instead of on a per-site basis), or may not be controllable at all.

In some cases, you may only be able to use a Master Preferences file to control the initial value for a setting, but the user may later change that value freely.

There’s a ton of great content about managing Edge in the Microsoft Edge Enterprise Documentation, including tables mapping Chrome and Edge Legacy policies to their Edge equivalents.


Seamless Single Sign-On

There are many different authentication primitives built into browsers. The most common include Web Forms authentication, HTTP authentication, client certificate authentication, and the new WebAuthN standard. Numerous different authentication frameworks build atop these, and many enterprise websites support more than one scheme.

Each of the underlying authentication primitives has different characteristics: client certificate authentication is the most secure but is hard to broadly deploy, HTTP authentication works great for Intranets but poorly for most other scenarios, and Web Forms authentication gives the website the most UI flexibility but suffers from phishing risk and other problems. WebAuthN is the newest standard and is not yet supported by most sites.

Real World Authentication Flows

Many Enterprises will combine all of these schemes, using a flow something like:

  1. User navigates to https://app.corp.example
  2. The web application determines that the user is not logged in
  3. The user is redirected to https://login.corp.example
  4. The login provider checks to see whether the user has any cached authentication tokens, e.g. using the cookies accessible to the login provider.
  5. If not, the login provider tries to fetch with a certificate filter specifying the internal CA root. If the user has a client certificate from the CA root, it is sent either silently or after a one-click prompt.
  6. If not, the login provider checks to see whether the user’s browser has any domain credentials using the HTTP Negotiate authentication scheme.
  7. If not, the login provider shows a traditional HTML form for login, ideally with a WebAuthN option that allows the user to use the new secure API rather than typing a password.
  8. After all of these steps, the user’s identity has been verified and is returned to the app.corp.example site.

In today’s post, I want to take a closer look at Step #6.

Silent HTTP Authentication

Unfortunately for our scenario, the HTTP Authentication scheme doesn’t support any sort of NoUI attribute, meaning that a server has no way to demand “Authenticate using the user’s domain credentials if and only if you can do so without prompting.”

WWW-Authenticate: Negotiate

And browsers’ HTTP Authentication prompts tend to be pretty ugly:

Depending upon client configuration and privacy mode, HTTP Authentication using the Negotiate (wrapping Kerberos/NTLM) or NTLM schemes may happen silently, or it may trigger the manual HTTP Authentication prompt.

So, at step #6, we’re stuck. If automatic HTTP authentication would’ve worked, it would be great– the user would be signed into the application with zero clicks and everything would be convenient and secure.

Load-Bearing Quirks

Fortunately for our scenario (unfortunately for understandability), there’s a magic trick that authentication flows can use to try HTTP authentication silently. As far as I can tell, it was never designed for this purpose, but it’s now used extensively.

To help prevent phishing attacks, modern browsers will prevent1 an HTTP authentication prompt from appearing if the HTTP/401 authentication response was for a cross-site image resource. The reasoning here is that many public platforms will embed images from arbitrary URLs, and an attacker might successfully phish users by posting on a message board an image reference that demands authentication. An unwary user might inadvertently supply their credentials for the message board to the third party site.

As noted in Chromium:

  if (resource_type == blink::mojom::ResourceType::kImage &&
      IsBannedCrossSiteAuth(request.get(), passed_extra_data.get())) {
    // Prevent third-party image content from prompting for login, as this
    // is often a scam to extract credentials for another domain from the
    // user. Only block image loads, as the attack applies largely to the
    // "src" property of the <img> tag. It is common for web properties to
    // allow untrusted values for <img src>; this is considered a fair thing
    // for an HTML sanitizer to do. Conversely, any HTML sanitizer that didn't
    // filter sources for <script>, <link>, <embed>, <object>, <iframe> tags
    // would be considered vulnerable in and of itself.
    request->do_not_prompt_for_login = true;
    request->load_flags |= net::LOAD_DO_NOT_USE_EMBEDDED_IDENTITY;

So, now we have the basis of our magic trick.

We use a cross-site image resource (e.g. into our login flow. If the image downloads successfully, we know the user’s browser has domain credentials and is willing to silently release them. If the image doesn’t download (because a HTTP/401 was returned and silently unanswered by the browser) then we know that we cannot use HTTP authentication and we must continue on to use the WebForms/WebAuthN authentication mechanism.

Update (Feb 2021): As of Chrome/Edge88, this magic trick will now fail if the user has configured their browser to “Block 3rd Party Cookies”, because the browser now treats a cross-origin authentication demand as if it were a cookie. Credentials for the cross-origin image will be omitted, and the browser will conclude that HTTP authentication is not available.


1 Note that this magic trick is defeated if you enable the AllowCrossOriginAuthPrompt policy, because that policy permits the authentication prompt to be shown.

Post-Script: Prompting for Credentials vs. Approving for Release

As an aside, the HTTP Authentication prompt shown in this flow is more annoying than it strictly needs to be. What it’s usually really asking is “May I release your credentials to this site?“:

…but for implementation simplicity and historical reasons the prompt instead forces the user to retype their username and password.

Beating Private Mode Blockers with an Ephemeral Profile

Back in 2018, I explained how some websites use various tricks to detect that visitors are using Private Mode browsers and force such users to log-in. The most common reason that such sites do this is that they’ve implemented a “Your first five articles are free, then you have to pay” model, and cookies or similar storage are used to keep track of the user’s read count.

The New Yorker magazine is one such site:

Unfortunately, such “Private Mode blockers” make it hard for those of us who use Private Mode for other reasons (I don’t want to leave any traces of my Beanie Baby shopping research!). Private Mode detectors typically trigger for Chromium-based browsers’ Guest Profile that you might be use when borrowing a trusted friend’s computer.

So, what’s a privacy-conscious user to do?

If you’re using Firefox, you can use that browser’s “Containers” feature to isolate such sites into a partitioned container such that trackers from the site cannot follow you around the web.

If you use Microsoft Edge, you might consider creating your own “Ephemeral” browser profile for browsing sites that block InPrivate:

After you create the new profile, visit its Settings page at edge://settings/clearBrowsingDataOnClose and configure all storage areas to be cleared every time you close the browser1:

Note: Chrome does not offer a Clear on Close list, but does offer a limited Clear cookies and site data when you quit Chrome option.

You can then adjust any other settings you like, for instance, adjusting Tracking Protection to Strict in edge://settings/privacy or the like.

Then when you want to visit a site that blocks InPrivate, you can either open your Ephemeral profile from your profile icon, or use the Open link as command on a hyperlink’s context-menu:

Over time, browsers will continue to work to make Private Mode detectors less reliable, but it’s unlikely that they’ll ever be perfect. Creating an ephemeral profile that clears everything on exit is a useful trick to combat sites which prioritize their business model needs over your privacy.


1 In Edge 85 and earlier, you must unfortunately close all browser windows (even from your main profile) to trigger the cleanup of your ephemeral profile; closing just the windows from the ephemeral profile alone is not enough. This bug was recently fixed in Edge 86.

Advanced Q&A

Q: How is this Ephemeral/ClearOnExit Profile different than a regular InPrivate Mode session?

A: There are a few key differences.

  1. InPrivate tries not to write anything to disk (although the OS memory manager might at any time decide swap process memory to the disk), while true profiles do not impose such a limitation. The “no disk write” behavior of Private Mode is the primary source of web-platform-observable differences in behavior that allow sites to build Private Mode detectors.
  2. By default, your default browser extensions do not load in InPrivate, but they can be configured to do so. In a different profile, you’ll have to install any desired extensions individually.
  3. By default, your credentials (usernames and passwords) do not autofill while InPrivate. In a different profile, your main profile’s credentials will not be available (and will be cleared on exit if configured to do so).
  4. InPrivate tabs do not perform Windows Integrated Authentication to Intranet sites automatically. Regular browser profiles do not have such a limitation.

Revealing Passwords

The Microsoft Edge browser, Edge Legacy, and Internet Explorer all offer a convenient mechanism for users to unmask their typing as they edit a password field:

Clicking the little eye icon disables the masking dots so that users can see the characters they’re typing:

This feature can be very useful for those of us who often mistype characters, and is especially important for users with various accessibility needs that can make error-free typing especially challenging. Keyboard users can hit ALT+F8 to toggle the reveal feature without using the mouse.

Nevertheless, Web Developers may disable this feature (for instance, if they offer their own version) by targeting the -ms-reveal pseudo element on an input type=password field:

.classNoReveal::-ms-reveal {
display: none;

If a site offers its own “reveal” feature, it should use CSS to hide the built-in feature to avoid confusing UI like this one:

Alternatively, sites may customize the Password Reveal Icon to better match their visual style.

Edge Legacy and Internet Explorer also respect a Windows policy (DisablePasswordReveal) that removes the password reveal button in various places throughout the system, including Edge Legacy and Internet Explorer. Some security configuration guides suggest setting this policy, arguing “Visible passwords may be seen by nearby persons, compromising them.” This is literally true; it is also true that such nearby persons might simply watch as the user’s fingers as they type in their password manually.

Notably, this Windows policy is not respected2 by Edge 79 and later, so we’ve had a few questions about that. I’d like to point out a few non-obvious characteristics of this feature that might assuage security concerns.

The most obvious attack that administrators are worried about is that a passerby might use this mechanism to steal auto-filled passwords from an unlocked, unattended computer. This concern is misplaced1: when the browser’s Password Manager autofills a password, the reveal icon is removed:

The PasswordInputType code is smart too– an attacker cannot get the icon to appear by simply adding or deleting a few characters, it only reappears after the user completely removes all of the characters in the input field. The icon is hidden if the field is modified by JavaScript, and it’s hidden if focus leaves the input field.

All of these protections mean that the Password Reveal icon is unlikely to be abusable in any meaningful way. Of course, typing passwords at all is an anti-pattern– use the Password Manager to mitigate phishing attacks, and eliminate the use of passwords wherever possible.


1 Notably, while concern about the reveal button is misplaced, it’s entirely possible to steal your own password using the Developer Tools or by running JavaScript from the omnibox.

2 In Edge 87, we added an Edge-specific Group Policy to suppress the reveal button. You shouldn’t use it.

Web Proxy Auto Discovery (WPAD)

Back in the mid-aughts, Adam G., a colleague on the IE team, used the email signature “IE Networking Team – Without us, you’d be browsing your hard drive.” And while I’m sure it was meant to be a bit tongue-in-cheek, it’s really true– without a working network stack, web browsers aren’t nearly as useful.

Background on Proxy Determination

One of the very first things a browser must do on startup is figure out how to send requests over the network. Typically, the host operating system already provides the transport (TCP/IP, UDP) and lower-level primitives, so the browser’s first task is to figure out whether or not web requests should be sent through a proxy. Until this question is resolved, the browser cannot send any network requests to load pages, sync profile information, update phishing blocklists, etc.

In some cases, proxy determination is simple— the browser is directly configured to ignore proxies, or to send all requests to a directly specified proxy.

However, for convenience and to simplify cases where a user might move a laptop between different networks with different proxy requirements, all major browsers support an algorithm called “Web Proxy Auto Discovery”, or WPAD. The WPAD process is meant to find and download a Proxy AutoConfiguration Script (PAC) for the current network.

The steps of the WPAD protocol are straightforward, if lengthy:

  1. Determine whether WPAD should be used, either by looking at browser settings or asking the host operating system if the browser is configured to match the OS setting.
  2. Ensure the network is ready.
  3. If WPAD is to be used, issue a DHCPINFORM query to ask for the URL of the PAC script to use.
  4. If the DHCPINFORM query fails to return a URL, perform a DNS lookup for the unqualified hostname wpad.
  5. If the DNS lookup succeeds, then the PAC URL shall be http://wpad/wpad.dat.
  6. Establish a HTTP(S) connection to discovered URL’s server and download the PAC script.
  7. If the PAC script downloads successfully, parse and optionally compile it.
  8. For each network request, call FindProxyForURL() in the PAC script and use the proxy settings returned from the function.

While conceptually simple, any of these steps might fail, and any failure might prevent the browser from using the network.


… or “Why on earth do I see Downloading proxy script… for a few seconds every time I start my browser!??!”

A Microsoft Edge feature team reached out to the networking team this week asking for help with an observed 3 second delay in the initialization of their feature. They observed that this delay magically disappeared if Fiddler happened to be running.

With symptoms like that, proxy determination is the obvious suspect, because Fiddler specifies the exact proxy configuration for browsers to use, meaning that they do not need to perform the WPAD process.

We asked the team to take an Edge network trace using the “Capture on Startup” steps. Sure enough, when we analyzed the resulting NetLog, we found almost exactly three seconds of blocking time during startup:

             --> new_config = Auto-detect
              --> source = "WPAD DHCP"
            --> net_error = -348 (ERR_PAC_NOT_IN_DHCP) 
              --> host = "wpad:80" 

Note: Timestamps [e.g. t=52] are shown in milliseconds.

Because the browser took a full three seconds to decide whether or not to use a proxy, every feature that relies on the network will take at least three seconds to get the data it needs.

So, where’s the delay coming from? In this case, the delay comes from two places: a two second delay for PAC_FILE_DECIDER_WAIT and a one second delay for the DNS lookup of wpad.

The two second PAC_FILE_DECIDER_WAIT [Step #2] is a deliberate delay that is meant to delay PAC lookups after a network change event is observed, to accommodate situations where the browser is notified of a network change by the Operating System before the network is truly “ready” to perform the DHCP/DNS/Download steps of WPAD. In this browser-startup case, we haven’t yet figured out why the browser thinks a network change has occurred, but the repro is not consistent and it seems likely to be a bug.

The (failing) DNS lookup [Step #3] might’ve taken even longer to return, but it timed out after one second thanks to an enabled-by-default feature called WPADQuickCheckEnabled.

This three second delay on startup is bad, but it could be even worse. We got reports from one Microsoft employee that every browser startup took around 21 seconds to navigate anywhere. In looking at his network log, we found that the wpad DNS lookup [Step #5] succeeded, returning an IP address, but the returned IP was unreachable and took 21 seconds to timeout during TCP/IP connection establishment.

What makes these delays especially galling is that they were all encountered on a network that does not actually need a proxy!


Beyond the time delays, each of these steps might fail, and if a proxy is required on the current network, the user will be unable to browse until the problem is corrected.

For example, we recently saw that [Step #7] failed for some users because the Utility Process running the PAC script always crashed due to forbidden 3rd-party code injection. When the Utility Process crashes, Chromium attempts to bypass the proxy and send requests directly to the server, which was forbidden by the Enterprise customer’s network firewall.

We’ve also found that care must be taken in the JavaScript implementation of FindProxyForURL() [Step #8] because script functions behave slightly differently across different browsers. In most cases, scripts work just fine across browsers, but sometimes corner cases are encountered that require careful handling.

Script Download

In Chromium, if a PAC script must be downloaded, it is fetched bypassing the cache.

Even if we were to comment out the LOAD_DISABLE_CACHE directive in the fetch, this wouldn’t allow reuse of a previously downloaded script file– my assumption is that the download is happening in a NetworkContext that doesn’t actually have a persistent cache, but I haven’t looked into this.

The PAC script fetches will be repeated on network change or browser restart.


WPAD is something of a security threat, because it means that another computer on your network might be able to become your proxy server without you realizing it. In theory, HTTPS traffic is protected against malicious proxy servers, but non-secure HTTP traffic hasn’t yet been eradicated from the web, and users might not notice if a malicious proxy performed an SSLStripping attack on a site that wasn’t HSTS preloaded, for example.

Note: Back in 2016, it was noticed that the default Chromium proxy script implementation leaked full URLs (including HTTPS URLs’ query strings) to the proxy script; this was fixed by truncating the URL to the hostname. (In the new world of DoH, there’s some question as to whether we might be able to avoid sending the hostname to the proxy at all).

Edge Legacy and Internet Explorer have a surprising default behavior that treats sites for which a PAC script returns DIRECT (“bypass the proxy for this request“) as belonging to your browser’s Intranet Zone.

This mapping can lead to functionality glitches and security/privacy risks. Even in Chrome and the new Edge, Windows Integrated Authentication still occurs Automatically for the Windows Intranet Zone, which means this WPAD Zone Mapping behavior is still relevant in modern browsers.

Chrome performing Automatic Authentication due to Proxy Bypass

Edge Legacy and Internet Explorer

Interestingly, performance and functionality problems with WPAD might have been less common for the Edge Legacy and Internet Explorer browsers on Windows 10. That’s because both of these browsers rely upon the WinHTTP Web Proxy Auto-Discovery Service:

This is a system service that handles proxy determination tasks for clients using the WinHTTP/WinINET HTTP(S) network stacks. Because the service is long-running, performance penalties are amortized (e.g. a 3 second delay once per boot is much cheaper than a 3 second delay every time your browser starts), and the service can maintain caches across different processes.

Chromium does not, by default, directly use this service, but it can be directed to do so by starting it with the command-line argument:


A Group Policy that matches the command-line argument is also available.


Prior to the enhancement of the WinHTTP WPAD Service, a feature called SmartWPAD was introduced in Internet Explorer 8’s version of WinINET. SmartWPAD caches in the registry a list of networks on which WPAD has not resulted in a PAC URL, saving clients the performance cost of performing the WPAD process each time they restarted for the common case where WPAD fails to discover a PAC file:

Cache entries would be maintained for a given network fingerprint for one month. Notably, the SmartWPAD cache was only updated by WinINET, meaning you’d only benefit if you launched a WinINET-based application (e.g. IE) at least once a month.

When a client (including IE, Chrome, Microsoft Edge, Office, etc) subsequently asks for the system proxy settings, SmartWPAD checks if it had previously cached that WPAD was not available on the current network. If so, the API “lies” and says that the user has WPAD disabled.

The SmartWPAD feature still works with browsers running on Windows 7 today.

Notably, it does not seem to function in Windows 10; the registry cache is empty. My Windows 10 Chromium browsers spend ~230ms on the WPAD process each time they are fully restarted.

Update: The WinINET team confirms that SmartWPAD support was removed after Windows 7; for clients using WinINET/WinHTTP it wasn’t needed because they were using the proxy service. Clients like Chromium and Firefox that query WinINET for proxy settings but use their own proxy resolution logic will need to implement a SmartWPAD-like feature optimize performance.

Disabling WPAD

If your computer is on a network that doesn’t need a proxy, you can ensure maximum performance by simply disabling WPAD in the OS settings.

By default (if not overridden by policy or the command line), Chromium adopts the Windows proxy settings by calling WinHttpGetIEProxyConfigForCurrentUser.

On Windows, you can thus turn off WPAD by default by using the Internet Control Panel (inetcpl.cpl) Connections > LAN Settings dialog, or the newer Windows 10 Settings applet’s Automatic Proxy Setup section:

Simply untick the box and browsers that inherit their default settings from Windows (Chrome, Microsoft Edge, Edge Legacy, Internet Explorer, and Firefox) will stop trying to use WPAD.

Looking forward

WPAD is convenient, but somewhat expensive for performance and a bit risky for security/privacy. Every few years, there’s a discussion about disabling it by default (either for everyone, or for non-managed machines), but thus far none of those conversations has gone very far.

Ultimately, we end up with an ugly tradeoff– no one wants to land a change that results in users being limited to browsing their hard drives.

If you’re an end user, consider unticking the “Automatically Detect Settings” checkbox in your Internet settings. If you’re an enterprise administrator, consider deploying a policy to disable WPAD for your desktop fleet.


Avoiding Unexpected Navigation

For over twenty years, browsers broadly supported two features that were often convenient but sometimes accidentally invoked, leading to data loss.

The first feature was that hitting backspace would send the user back one page in their navigation history. This was convenient for those of us who keep our hands on the keyboard, but could lead to data loss– for instance, when filling out a web form, if focus accidentally left a text box, hitting backspace could result in navigating away from the form. Smart websites would warn you with an OnBeforeUnload handler, and some browsers tried to restore the contents of the form if the user understood what happened and hit “forward”, but these protections are imperfect and might not work for all forms.

One of the IE browser UI leads complained about this behavior annually for a decade, and users periodically howled as they lost work. Finally, circa 2016, this feature was removed from Chrome and Microsoft Edge followed in 2018. If you happened to love the old behavior and accept the risk of data loss, you can restore it via extension or in Edge 86, via the edge://flags/#edge-backspace-key-navigate-page-back flag.

The second feature was drop to navigate. Dragging and dropping a file into the browser’s content area (the body of the page) would, (unless the page’s JavaScript was designed to handle the drop, e.g. to upload it or process it locally), navigate to that local file in the current tab. Some folks like that behavior– e.g. web developers loading HTML files from their local filesystem, but it risks the same data loss problem. If a web page doesn’t accept file uploads via drag/drop, the contents of that page will be blown away by navigation. Back in 2015, a bug was filed against Chromium suggesting that the default behavior was too dangerous, and many examples were provided where the default behavior could be problematic. Yesterday, I landed a tiny change for Chromium 85 [later merged to v84] such that dropping a file or URL into the content area of a tab will instead open the file in a new tab:

Dropping in the content area now opens it in a new tab:

If you do want to replace the content of the tab with the dropped file, you can simply drag/drop the file to the tab strip.

A small white arrow shows you which tab’s content will be replaced:

Dropping the file between tabs on the tab strip will insert a new tab at the selected location:

Chrome (85.0.4163/v84) and Microsoft Edge (85.0.541) include this change; it was also later merged to v84. Microsoft Edge Legacy didn’t support drop to navigate. Firefox still has the old behavior; the closest bug I could find is this one. Safari 13.1.1 still has the old behavior and replaces the content of the current tab with the local file. Safari Tech Preview 13.2 Release 108 instead navigates the tab to an error page (NSPOSIXErrorDomain:1 Operation not permitted”).


Browser Basics: User Gestures

The Web Platform offers a great deal of power, and unfortunately evil websites go to great lengths to abuse it. One of the weakest (but simplest to implement) protections against such abuse is to block actions that were not preceded by a “User Gesture.” Such gestures (sometimes more precisely called User Activations) include a variety of simple actions, from clicking the mouse to typing a key; each interpreted as “The user tried to do something in this web content.”

A single user gesture can unlock any of a surprisingly wide array of privileged (“gated”) actions:

  • Allow a popup window to open
  • Allow an Application Protocol to be invoked
  • Allow an OnBeforeUnload dialog box to show
  • Allow the Vibration API to vibrate the device
  • Allow script to take the window fullscreen
  • Allow the password manager to fill the username/password into the page in a way that JavaScript can see them
  • Allow the page to prompt the user for a file to upload
  • Impact the behavior of file downloads (e.g. prompting)
  • and many more

So, when you see a site show a UI like this:

…chances are good that what they’re really trying to do is trick you into performing a gesture (mouse click) so they can perform a privileged action– in this case, open a popup ad in a new tab.

In terms of which actions can cause a gesture, the list is surprisingly limited, and includes mousedown (but not mouseup/click):

// Returns |true| if |type| is the kind of user input that should trigger user interaction observers.
bool IsUserInteractionInputType(blink::WebInputEvent::Typetype) {
// Ideally, this list would be based more off of
// return type ==
blink::WebInputEvent::Type::kMouseDown ||
type == blink::WebInputEvent::Type::kGestureScrollBegin ||
type == blink::WebInputEvent::Type::kTouchStart ||
type == blink::WebInputEvent::Type::kRawKeyDown; }

Some gestures are considered “consumable”, meaning that a single user action allows only one privileged action; subsequent privileged actions require another gesture. Web Developers do not have unlimited time to consume the action: In Chrome, when you click in a web page, the browser considers this “User Activation” valid for five seconds (as of Februrary 2019) before it expires. Here’s a simple test.

Unfortunately, even this weak protection is subject to both false positives (an unwanted granting of privilege) and false negatives (an action is unexpectedly blocked).

You can learn more about this topic (and the complexity of dealing with nested frames, etc) in the original Chromium User Activation v2 spec, and the User-Activation section of HTML5.


A bit of GREASE keeps the web moving

For the first few years of the web, developers pretty much coded whatever they thought was cool and shipped it. Specifications, if written at all, were an afterthought.

Then, for the next two decades, spec authors drafted increasingly elaborate specifications with optional features and extensibility points meant to be used to enable future work.

Unfortunately, browser and server developers often only implemented enough of the specs to ensure interoperability, and rarely tested that their code worked properly in the face of features and data allowed by the specs but not implemented in the popular clients.

Over the years, the web builders started to notice that specs’ extensibility points were rusting shut– if a new or upgraded client tried to make use of a new feature, or otherwise change what it sent as allowed by the specs, existing servers would fail when they saw the encountered the new values. (Formally, this is called ossification).

In light of this, spec authors came up with a clever idea: clients should send random dummy values allowed by the spec, causing spec-non-compliant servers that fail to properly ignore those values to fail immediately. This concept is called GREASE (with the backronym “Generate Random Extensions And Sustain Extensibility“), and was first implemented for the values sent by the TLS handshake. When connecting to servers, clients would claim to support new ciphersuites and handshake extensions, and intolerant servers would fail. Users would holler, and engineers could follow up with the broken site’s authors and developers about how to fix their code. To avoid “breaking the web” too broadly, GREASE is typically enabled experimentally at first, in Canary and Dev channels. Only after the scope of the breakages is better understood does the change get enabled for most users.

GREASE has proven such a success for TLS handshakes that the idea has started to appear in new places. Last week, the Chromium project turned on GREASE for HTTP2 in Canary/Dev for 50% of users, causing connection failures to many popular sites, including some run by Microsoft. These sites will need to be fixed in order to properly load in the new builds of Chromium.

// Enable "greasing" HTTP/2, that is, sending SETTINGS parameters with reserved identifiers and sending frames of reserved types, respectively. If greasing Frame types, an HTTP/2 frame with a reserved frame type will be sent after every HEADERS and SETTINGS frame. The same frame will be sent out on all connections to prevent the retry logic from hiding broken servers.
NETWORK_SWITCH(kHttp2GreaseSettings, "http2-grease-settings") NETWORK_SWITCH(kHttp2GreaseFrameType, "http2-grease-frame-type")

One interesting consequence of sending GREASE H2 Frames is that it requires moving the END_STREAM flag (recorded as fin=true in the netlog) from the HTTP2_SESSION_SEND_HEADERS frame into an empty (size=0) HTTP2_SESSION_SEND_DATA frame; unfortunately, the intervening GREASE Frame is not presently recorded in the netlog.

You can try H2 GREASE in Chrome Stable using command line flags that enable GREASE settings values and GREASE frames respectively:

chrome.exe --http2-grease-settings
chrome.exe --http2-grease-frame-type

Alternatively, you can disable the experiment in Dev/Canary:

chrome.exe --disable-features=Http2Grease

GREASE is baked into the new HTTP3 protocol (Cloudflare does it by default) and the new UA Client Hints specification (where it’s blowing up a fair number of sites). I expect we’ll see GREASE-like mechanisms appearing in most new web specs where there are optional or extensible features.