Update 2F Spec Assumptions

[OisinNolan]

• Step 2F has 2 spec assumptions relating to whitespace, although these assumptions have been clarified thanks to the Web Platform Tests (see http://wpt.live/accname/name_file-label-inline-block-elements-manual.html) and this issue: Consolidating issues for handling whitespace where the spec algorithm breaks down w3c/accname#16 by Bryan Garaventa.
• The spec assumptions for 2F should be updated in accordance with our current approach to whitespace, namely that we now consider whether an element is 'inline' or 'block', and also that we consider the 'display' value for CSS pseudo elements.