The section on ::first-letter says nothing about whitespace, and a css-backgrounds WPT test even tests that whitespace is not selected by ::first-letter. However, all implementations fail this test:

• Edge selects whitespace no matter if it is followed by a non-whitespace character or not
• Firefox only selects whitespace that is not followed by a non-whitespace character
• Chrome (and I assume WebKit) only selects whitespace that is followed by a non-whitespace character

Given that all implementations select whitespace of some sort in ::first-letter, but each does it differently, it is probably appropriate to clarify this in the spec.