HTMLImageElement.src in about:blank documents

[josepharhar]

What is the issue with the HTML Standard?

Chrome, Firefox, and Safari all have special behavior in HTMLImageElement.src (and other URL-related reflected attributes?) which "completes" the URL from the string provided in the actual attribute.

At http://example.com, we have the following behavior which is compatible across these browsers:

// At http://example.com/
const img = document.createElement('img');
img.src = '/foo';
img.src; // returns "http://example.com/foo"
img.getAttribute('src'); // returns "/foo"

I'm not sure if this behavior is specified or not. In chrome we have a "URL" IDL attribute in the IDL for src which generates code to do this behavior.

In about:blank documents, however, Chrome returns an empty string, I think because it considers the URL to not be valid. Firefox and Safari just return what was assigned into the attribute:

// At about:blank
const img = document.createElement('img');
img.src = '/foo';
// Chrome returns ""
// Firefox+Safari return "/foo"
img.src;

What is the desired behavior here? Should I just make Chrome return whatever the actual attribute value is for non-valid URLs, like in about:blank documents?

[domenic]

This is well-specified here, under the "If a reflected IDL attribute has the type USVString" case. It has nothing to do with about:blank documents in particular, just about URL parsing: parsing /foo against about:blank results in a parse failure. Because the result of step 4.2 is failure, we fall back to step 5, so Firefox and Safari's behavior is correct.

In particular, this means the bug is in Chromium's URL reflected attribute thing. This maybe the source of some of Chrome's Interop 2023 failures on URLs. /cc @hayatoito No, nevermind, HTMLAnchorElement does not use [Reflect, URL] in this way.

I hope this helps!

[josepharhar]

Thanks Domenic! I opened a crbug here: https://bugs.chromium.org/p/chromium/issues/detail?id=1493146
I couldn't easily figure out how the generated bindings code works myself, so hopefully someone else can help out.