[css-text-decor-4] Default end value of `text-decoration-trim` when a single length is specified?

[cdoublev]

Can you please clarify what is the value of the end point when a single length is specified?

Name: text-decoration-trim
Value: <length>{1,2} | auto

0 seems a sensible option. The inverse of the start point may be more intuitive, eg. -1em when the start point is 1em. Or auto, but the syntax does not allow to explicitly declare it.

[jfkthame]

It seems pretty clear to me (though arguably could be made more explicit) that the intent of the spec is that if only a single <length> value is specified, it applies to both the start and end of the decoration.

This means that a single <length> will either shorten (positive) or lengthen (negative) the line at both ends; to shift the line, it will be necessary to provide two values with opposite signs, and to adjust only one side, an explicit zero must be provided for the other.

[cdoublev]

The property name includes "trim" so it may be suprising that the line decoration is actually shifted when a value is omitted. And authors might think that the first value trims to the left, and the second to the right.

In my experience, default values are almost always defined explicitly, although they often default to the first value specified. If I had been 100% sure of the expected default value when I created this issue, I would have made a PR to make it explicit. Do you want me to do that now?

[jfkthame]

The property name includes "trim" so it may be suprising that the line decoration is actually shifted when a value is omitted. And authors might think that the first value trims to the left, and the second to the right.

But it's not shifted when a value is omitted. With a single positive value, it's trimmed (shortened) at both ends; with a single negative value it's "negatively trimmed" (i.e. extended) at both ends.

[jfkthame]

The spec draft currently says:

If two component values are given, the first applies to the start and the second to the end.

Would you find it helpful if we added something like:

If a single <length> value is given, it applies to both the start and end of the decoration line. Note that when the same value is applied to both ends, they will be adjusted in opposite physical directions (i.e. both inset if the value is positive, both outset if it is negative).

to make things clearer?

[cdoublev]

I misread your comment. Sorry for the confusion. I think your additional text would be great. 😊