[cssom] "case-sensitive match for a supported CSS property"?

[SimonSapin]

In each of:

• https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty
• https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setpropertyvalue
• https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setpropertypriority

… one of the steps is:

If property is not a case-sensitive match for a supported CSS property, terminate this algorithm.

… with a link to:

https://drafts.csswg.org/cssom/#supported-css-property

The term supported CSS property refers to a CSS property that the user agent implements, including any vendor-prefixed properties, but excluding custom properties.

(Emphasis added.)

This does not match the behavior of at least Firefox and Chromium, where the matching is (probably ASCII) case-insensitive, and custom properties are not excluded:

document.documentElement.style.setProperty("Color", "green")
document.documentElement.style.setProperty("--a", "b")
assert_equals(document.documentElement.style.getPropertyValue("color"), "green");
assert_equals(document.documentElement.style.getPropertyValue("--a"), "b");

[SimonSapin]

The case-sensitive match makes sense since property was ASCII-lower-cased in a previous step. This leaves excluding custom properties.

By the way, custom property names should not be lower-cased:

document.documentElement.style.setProperty("--A", "b");
assert_equals(document.documentElement.style.getPropertyValue("--A"), "b");
assert_equals(document.documentElement.style.getPropertyValue("--a"), "");