[css-values] Make slashes implicitly omissible like commas

[SebastianZ]

For commas in values there is a definition that says that they are implicitly omissible when

• all items preceding the comma have been omitted
• all items following the comma have been omitted
• multiple commas would be adjacent (ignoring white space/comments), due to the items between the commas being omitted.

I believe the same logic should apply to slashes within values.

One explicit case for that is currently discussed in #7142.

Sebastian

[tabatkins]

No, this is impossible - the slash is absolutely required in several properties already to avoid ambiguity. In particular, 'font' requires it to distinguish a font-size from a line-height, and 'border-image' requires it to distinguish all four of its longhand components.

[SebastianZ]

This change wouldn't touch existing syntaxes because they don't allow leading or trailing slashes nor multiple subsequent slashes without values in between them.

font requires <'font-size'> to be set at the left side of the slash and <'line-height'> on the right side. border-image requires either <'border-image-source'> or <'border-image-slice'> on the left and <'border-image-width'> and <'border-image-outset'> on the right side.
I.e. in those cases the slash is required and the rules mentioned above do not apply.

Sebastian

[tabatkins]

Nope, border-image allows syntax like 0 // 0 (slice and outset specified, width unspecified between them), which would run afoul of the "two commas next to each other" rule.

I misremembered 'font' - the font-size is indeed always required, so the / <'line-height'> wouldn't run afoul of these rules.

Regardless, tho, border-image has an irreconcilable conflict.

[SebastianZ]

Oh, you're right. 😞 I misinterpreted the syntax of border-image and didn't realize that 0 // 0 is valid.

Though the main point why I came up with this idea was to avoid leading or trailing slashes in values. I'll open a related issue to discuss that.

Sebastian