Browsers handle ideographic space differently during line-breaking

[r12a]

Browsers differ in how they handle line breaks for IDEOGRAPHIC SPACE (U+3000).

[r12a]

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[xfq]

• Issue for collecting requirements: Ideographic spaces at the beginning or end of lines #411
• Line_Break property: BA (tailorable)
• Spec issues: [css-text] Ideographic spaces at the beginning or end of lines csswg-drafts#6903
• Test case