[css-text-decor-4] Disallow negative spread values on text-shadow

[smfr]

This thread describes some difficulties in implementing spread on text-shadow. The gist is that spread on box-shadow can be implemented via changing rectangle sizes and adjusting corner radii, but spread on text-shadow would have to be implemented by shadowing the text rendered with an enlarged stroke, which could lead to unexpected shadowing of parts of glyphs in some fonts (I saw issues with Zapfino).

In addition, the stroke-enlarging implementation doesn't allow for negative spread, so I suggest we disallow negative spread on text-shadow.

[LeaVerou]

I'm fine with that, most use cases for text-shadow spread are to add outline-like things, which require positive spread.

[smfr]

Then the question is "parse as invalid" or "clamp to 0". I suggest "clamp to 0".

[LeaVerou]

If we clamp to 0, we can never change that behavior, nor can it be tested via @supports. It also makes it harder to debug. What's the argument for clamping to 0?

[smfr]

Less surprising than parsing as invalid (esp if you copy a value from box-shadow) but I don't feel strongly.

[tabatkins]

I'm in favor of making it a non-negative range, for the reasons Lea gives. Negative numbers should be invalid if they aren't going to do anything, rather than being silently clamped to a different behavior.

[bradkemper]

Couldn't you stroke the alpha channel with a white stroke to create the choke?

Or could you use an erode filter on it?

[bradkemper]

Also, Wow! That thread is 10 years old. I'm happy we are returning to this.

Are you also looking at supporting the inset keyword in text-shadow?

[SebastianZ]

Are you also looking at supporting the inset keyword in text-shadow?

Just for reference, adding inset to the spec. is discussed in #6074.

Sebastian

[css-meeting-bot]

The CSS Working Group just discussed Text Shadow Negative Spread, and agreed to the following:

• RESOLVED: Negative spread values on text-shadow are invalid

The full IRC log of that discussion

<fantasai> Topic: Text Shadow Negative Spread
<fantasai> github: https://github.com//issues/6971
<fantasai> fantasai: Proposal is to disallow negative spread values on text-shadow by making them invalid
<emeyer> +1 from me on making them invalid
<fantasai> astearns: Any objections?
<TabAtkins> +1
<fantasai> smfr: Minor suprise for authors, because box shadow and text shadow have different behavior, but I'm OK with it
<fantasai> RESOLVED: Negative spread values on text-shadow are invalid