[css-values-4] `snap a length as a border width`: meaning of "Assert: len is non-negative."?

[romainmenke]

see: https://drafts.csswg.org/css-values-4/#snap-a-length-as-a-border-width

Assert: len is non-negative.

What is the meaning/implication of Assert in this context?

Is the value invalid when negative?

Because step 2 explicitly has "do nothing" I am assuming it makes the value invalid, but this seems weird.

If the intention is to do nothing, it might be better to write as:

1. If len is negative, do nothing.
2. If len is an integer number of device pixels, do nothing.
...

@tabatkins

[Loirooriol]

It means that whatever invokes the "snap a length as a border width" algorithm should make sure that len is non-negative.

[romainmenke]

Thanks!

In that case this is somewhat related to: #13794

Otherwise, identical to nearest, except that if A is positive (non-zero) and one of lower B or upper B is zero, choose the non-zero one, then snap as a border width.

This can indeed be read as an assert that A must be positive and that in other cases there is no snapping to border width.

[fantasai]

It means that whatever invokes the "snap a length as a border width" algorithm should make sure that len is non-negative.

... This doesn't seem great? It should snap a negative number snap(N) same as -snap(|N|).

[fantasai]

Fixed in c196082

@romainmenke @Loirooriol Please re-open if there's a problem remaining, but we think this handles it.