The ::first-line pseudo-element can only be styled by a limited set of properties. Those properties are:

• all font properties
• the color and opacity properties
• all background properties
• any typesetting properties that apply to inline elements
• all text decoration properties
• the ruby-position property
• any inline layout properties that apply to inline elements
• any other properties defined to apply to ::first-line by their respective specifications

See https://drafts.csswg.org/css-pseudo-4/#first-line-styling and https://developer.mozilla.org/en-US/docs/Web/CSS/::first-line#allowable_properties.

Sebastian

Note that some of the accepted properties like 'margin-trim' are not yet supported by Gecko. Some are supported by the engine but not yet for the ::first-line pseudo-element like 'padding'.
According to the specification, those are valid within ::first-line, though. See https://www.w3.org/TR/css-pseudo-4/#first-line-styling and its referenced specifications as well as https://www.w3.org/TR/css-box-4/#placement. There might even be other properties supported, though the specs are not totally clear about that.

For reference, Julian pointed out that the spec. says this:

User agents may apply other properties as well except for the following excluded properties:

• writing-mode
• direction
• text-orientation

Therefore, we now only mark those three properties as inactive instead of every property besides the ones listed in the initial comment.

Sebastian

https://hg.mozilla.org/mozilla-central/rev/a6e6cc51fc00

This might be worth being mentioned in https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/118 together with the changes of bug 1583912 and bug 1583906.

Sebastian