A run-in sequence is defined as

a maximal sequence of consecutive sibling run-in boxes and intervening white space and/or out-of-flow boxes.

This definition does not forbid empty run-in sequences. For example, consider

<span>Foo</span><span>Bar</span>

I could say that between the two <span>s there is an empty run-in sequence, which should be wrapped inside an anonymous block box according to the run-in layout rules. Then Foo and Bar would appear in different lines. This is bad.

I would change the definition to "a maximal non-empty sequence [...]"