Edge Interop Issues

As we finish up the next release of Windows 10 (Fall 2018), my team is hard at work triaging incoming bugs.

Many such bugs take the form “Edge does the wrong thing for this page. works okay.

This post is designed to be an (ever-growing) index of some of the behavioral deltas that are the root cause of such issues:

Edge doesn’t allow navigation to DATA urls, even when they’d otherwise be converted to file downloads.

Using pushState or replaceState with |undefined| as the URL argument shows “undefined” in the Address box in Edge/IE but not Chrome or Firefox.

IE/Edge strip the Content-Encoding header from a compressed response; Firefox and Chrome leave the header in. For XmlHttpRequest’s getAllResponseHeaders, IE and Firefox maintain the case of HTTP Response header names while Chrome/Edge/Safari do not.

Chrome recognizes that a file with a .JSON extension has the type application/json (and vice versa) while IE/Edge only recognize that when the registry is configured with that mapping.

Chrome includes a hack that works around certificates that do not exactly match the domain on which they are served. Firefox, Edge, and IE do not include this hack, leading to a Certificate Name Mismatch Error when loading:


Edge does not fully support the URL standard, meaning that URLs of the form http:/ (note the missing slash) do not work as expected.

Edge and IE do not allow navigation to HTTP URLs containing a UserInfo component. Other browsers currently (reluctantly) allow this syntax.

Edge RS5 introduces support for Web Authentication specification (in order to support FIDO2 tokens). That specification extends the Credential Management API with new methods, so the navigator.credentials object now exists. However, Edge does not implement the navigator.credentials.preventSilentAccess() method and attempting to call it will cause an exception due to the missing method. (Edge always prevents silent access, so a future implementation of this method will simply fulfill the promise immediately).

(…to be continued…)



The Sad State of HAR

Spring 2017 Update: Some of these issues have been fixed.

The HTTP Archive Format (HAR) was designed to allow tools to exchange network traffic using a standard format; this format is akin to Fiddler’s Session Archive Zip format but is supported natively by browser developer tools. Unlike SAZ files, it is not compressed by default, and often includes redundant text to simplify parsers.

Unfortunately, none of the four major browsers (IE, Edge, Chrome, Firefox) generates HAR correctly.

Internet Explorer 11 and below: Generates the file in XML instead of the proper JSON, due to a misreading of the specification. The export is also limited by numerous bugs in the F12 Network Capture tool, including missing data and misrepresentation of certain response types (e.g. 304s).

Firefox 45: Attempts to store GZIP’d response bodies as text.

Fails to include the encoding=”base64″ attribute when storing binary bodies using base64 encoding.

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Microsoft Edge: Fails to include the encoding=base64 token when storing binary bodies using base64 encoding.

Chrome 47: “Save as HAR with content” doesn’t save the content.

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Unfortunately, there seems to be little effort to clean these problems up; the IE bug is at least four years old, the Edge bug is at least four months old. I filed bugs on Chrome and Firefox after failing to find any duplicates.

The HAR format specification itself has a number of shortcomings that have yet to be corrected, for instance:

  • No specified way to encode binary request bodies
  • No specified way to encode WebSocket messages

Perhaps the name of HAR spec-author Jan Odvarko’s blog is prescient: Software is hard.