Bug 262762

Background:
  CSS Text Level 4 <https://www.w3.org/TR/css-text-4/> adds support for percentage values of word-spacing and letter-spacing. These inherit as percentages, so that they are always relative to the currently-effective font-size.

Current Status:
  WebKit has support for percentage values of word-spacing (but not calc() values) but calculates them against the width of a space instead of the font-size, and doesn't inherit them properly; and has no support for percentage letter-spacing.

Proposal:
  Re-implement percentage values of word-spacing per spec, add support for percentage values of letter-spacing, and implement support for calc().

Compat Risks:
  Firefox also implements percentage word-spacing in a similar way to WebKit, but Chrome doesn't implement percentage word-spacing, so it's unlikely that Web content depends on it. There could be some lock-in in single-engine environments, but because the current implementation doesn't implement inheritance properly, the benefit of using % over fixed values is not fully realized in the current implementation, so the incentive to use them over 'em' values (which have existed since CSS1) is limited.