Re: [csswg-drafts] [css-inline-3] Clarification needed: Line box height can exceed a fixed line-height with mixed fonts (#12764)

Thank you for the quick response and for confirming the technical behavior. I'd like to clarify the core of the issue from a web developer's perspective.

The confusion stems from a common interpretation of the specification, which is often summarized by the following principle:

> "Metrics from fonts other than the first available font only impact the layout bounds of an inline box with line-height: normal."

Our reproducible example demonstrates that this principle is not always true, which is the source of significant developer confusion.

**Expectation vs. Reality**

- **Expectation (based on the principle):** A developer would reasonably expect that for our example with a fixed line-height: 1.2, the fallback font's metrics would have no impact on the layout bounds, resulting in a final div height of exactly 120px.
- **Reality:** The actual computed height is 128px, proving that the secondary font's metrics do impact the layout bounds, even with a fixed line-height.

**Request for Clarification**

This shows that the simplified principle is misleading. We believe the specification could be significantly improved by adding an explicit note to the definition of line-height to address this non-intuitive outcome. For example:

> "Note: The line-height property is used to calculate the height of individual inline boxes. However, the final line box's height is the geometric sum of the highest and lowest points of all aligned boxes. When a line contains content from multiple fonts with different metrics, this may result in a final line box that is taller than the computed line-height."

Mentioning that the upcoming line-fit-edge property is intended to provide more direct control over this behavior.

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Received on Friday, 12 September 2025 12:42:27 UTC