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.
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.