Fix a broken internal reference.

Referencing step 10, substep 12 isn't accurate any longer.
This creates a less fragile reference.
Was probably broken since ea580809ffe50bec627317bd0216f952b18d4552 .
Merge pull request #4 from silviapfeiffer/CSS-ref

Fix a broken internal reference.

The use of "initial containing block" appears to be incorrect, bug filed:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24326