The section on Raised and Sunken Caps specifies that the initial letter's margin box contributes to the size of the containing block. However, it's not clear on the mechanism that's used for this.

In particular, it should be clear about whether this works by:

• increasing the size of the first line box (on its block-start side), or
• adding space between the content box of the block and its first line box, or
• something else.

It should also presumably be clear that it increases the size of the block only if the margin-box of the initial letter would (based on its alignment) protrude above the top of the first line box.

The difference is detectable, at a minimum, by testing vertical-align: top elements in the first line.