What does font-size really correspond to?
Your understanding of the W3C references is correct, but your understanding of the red box is not. The red box does not depict the em-square. Its height is the sum of the ascent and descent from the metrics.
fontmetrics
Meaning of top, ascent, baseline, descent, bottom, and leading in Android’s FontMetrics
Let’s first review what the documentation says: Top – The maximum distance above the baseline for the tallest glyph in the font at a given text size. Ascent – The recommended distance above the baseline for singled spaced text. Descent – The recommended distance below the baseline for singled spaced text. Bottom – The maximum … Read more