[css-overflow] Bidi handling for text-overflow and line-clamp ellipses

[jja08111]

The CSS Overflow Module Level 4 draft specifies inconsistent bidirectional (bidi) handling for the ellipsis generated by text-overflow versus the one generated by the line-clamp property.

1. text-overflow: ellipsis: The specification implies that the ellipsis is part of its paragraph for bidi purposes. Implementations typically treat it as inheriting the direction of the line it is attached to.
2. line-clamp (block-ellipsis): The specification for block-ellipsis states that it behaves as if it had unicode-bidi: plaintext.

This leads to two different implementation approaches for what is visually the same feature:

• text-overflow ellipsis inherits the parent's direction.
• line-clamp ellipsis uses plaintext logic to determine its direction.

This difference in behavior is surprising and seems unintentional. We'd like to ensure this behavior.

Context: https://chromium-review.googlesource.com/c/chromium/src/+/6817921/comment/c4178435_9956b836/

cc. @andreubotella @kojiishi

[kojiishi]

/cc @jfkthame Firefox is shipping the <string> option [caniuse].