Full support for the CSS `white-space` property

[nicoburns]

The CSS white-space property is defined in sections 3 and 4 of the css-text-4 specification: https://drafts.csswg.org/css-text-4/#white-space-property. It is a very important property for accurately reproducing how browsers lay out text. And can also be used to account for differences between desired behaviour between editable and non-editable text around how whitespace is "hung" at the end of lines.

(the emojis in this issue were not generated by AI)

It is considered a "shorthand" property that decomposes into the following longhands:

✅ text-wrap-mode

This property has two variants: wrap and nowrap. nowrap disables wrapping entirely.

enum TextWrapMode {
     Wrap,
     NoWrap,
}

Status: fully implemented.

⚠️ white-space-collapse

This property controls:

• ⚠️ How whitespace is collapsed
• ❌ How whitespace at the end of the line is "hung" (or not hung)

enum WhiteSpaceCollapse {
      Collapse,
      Discard,
      Preserve,
      PreserveBreaks,
      PreserveSpaces,
      BreakSpaces,
}

See spec (https://drafts.csswg.org/css-text-4/#white-space-collapsing) for the meanings of each variant.

Status: we currently implement only the Collapse and Preserve variants in a somewhat hacky way in the TreeBuilder, and we also only implement the "white space collapsing" behaviour, and do not implement the different "whitespace hanging" behaviours (implementing the latter will require us to pass WhiteSpaceCollapse into layout as a style which we do not currently do).

See also: discussion on #485 which attempted fixes for whitespace hanging behaviour.

⚠️ white-space-trim

Controls "whitespace trimming" at the beggining and end of spans.

enum WhiteSpaceCollapse {
      DiscardBefore,
      DiscardAfter,
      DiscardInner,
}

Status: some basic trimming is implemented hackily in the TreeBuilder, but there are bugs in the current implementation. And it also doesn't allow full control over the trimming using a style.

[conor-93]

I've been able to get some of the requirements for white-space-collapse working here, though these are hard-coded for now. This could be used as a starting point (or just inspiration) for a full implementation.

[DJMcNab]

To write down some discussion from last week's office hours (#office hours > Text 2026-07-07), a significant amount of this is:

1. HTML specific
2. Possible to encode with simple rewrite rules.

My initial position is that we only need to implement the parts which aren't possible to handle with pure text transforms, which is primary BreakSpaces vs pre-wrap (note that I haven't wrapped my head around the decomposed forms yet).