Maybe ensure there is a path item after the Windows drive letter path item

[zcorpan]

https://url.spec.whatwg.org/commit-snapshots/26caf4927f6c95ba4f56d701ecd8a52fdb664982/#path-state

Otherwise, if buffer is not a single-dot path segment, run these subsubsteps:​

If url’s scheme is "file", url’s path is empty, and buffer is a Windows drive letter, run these subsubsubsteps:​

If url’s host is non-null, syntax violation.

Set url’s host to null and replace the second code point in buffer with ":​".

This is a (platform-independent) Windows drive letter quirk.

Append buffer to url’s path.

From web-platform-tests/wpt#4382 (comment)

Parsing //d: with base file:///C:/a/b (or just parsing file:///d:) currently gives a single string "d:" in the path, so serializing does not include a trailing slash. Maybe it should?

http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4769

No trailing slash:

Edge 13 and IE 9..11, Safari 9.1 and TP (WebKit 12603.1.16), Firefox Nightly 52.0a1 (2016-11-14) and 53.0a1 (2017-01-02) on Mac OS X, Chrome Canary 57.0.2970.0 on Mac OS X

With trailing slash:

Chrome 50.0 on Windows 10, Firefox 44.0 on Windows 10

(I don't know if there is a difference between platforms here for Chromium and Gecko, or if it changed between recent versions.)

[annevk]

Aligning with Edge seems good. Let's close this.