[css-display] Run-in sequences should not be allowed to be empty

[Loirooriol]

A run-in sequence is defined as

a maximal sequence of consecutive sibling run-in boxes and intervening white space and/or out-of-flow boxes.

This definition does not forbid empty run-in sequences. For example, consider

<span>Foo</span><span>Bar</span>

I could say that between the two <span>s there is an empty run-in sequence, which should be wrapped inside an anonymous block box according to the run-in layout rules. Then Foo and Bar would appear in different lines. This is bad.

I would change the definition to "a maximal non-empty sequence [...]"

[fantasai]

There are no run-in boxes there, therefore there can't be intervening white space and/or out-of-flow boxes.

[Loirooriol]

@fantasai It seems you didn't understand. I was not talking about intervening white space and/or out-of-flow boxes. I was talking about the "maximal sequence of consecutive sibling run-in boxes". A "a maximal sequence of consecutive sibling run-in boxes" can have zero, one, two or whatever countable number of consecutive sibling run-in boxes. I want to forbid the zero case. At least there should be some run-in box in the sequence.

[fantasai]

I don't think it's reasonable to consider nothing as a “maximal sequence of ... boxes”. We write specs for reasonable readers, not antagonistic ones. :/ I can throw “non-zero” in that list of adjectives there, but is it really necessary?

[Loirooriol]

After filing this issue I started noticing other specs that say "a sequence of ..." and almost nowhere it made sense to include empty sequences. Adding "non-empty" everywhere could be needed technically but I agree in practice it would be noise. Maybe it would be better to define in some boilerplate or meta spec that when a CSS spec says "sequence", it must be non-empty (unless otherwise specified).

Or well, this is a bit nitpicky, it's not a big problem if you leave it as it is.

[fantasai]

Yeah, I think it's a little excessively nitpicky. :) Empty sequences are equivalent to nothing, so should behave as nothing! Closing out as no change.