* Hygiene: clean up the xhtml namespace attribute; this is an artefact of the `serializeToString` method which we are using (I think) to be consistent with whitespace and to clean up invalid attributes. I'm removing as was confused as am adding tests related to doctypes
* Record when a document is in `compatMode` and trigger this mode on the iframe upon replay
https://developer.mozilla.org/en-US/docs/Web/API/Document/compatMode
the included DOCTYPE was picked up from https://stackoverflow.com/questions/18976213/ - there may be better ways of triggering compatMode
* Don't write an extra DOCTYPE if there's one already present in the snapshot. Rely instead on whatever doctype is there to trigger the BackCompat mode
* Modify to write the correct doctype if we can sniff xhtml - don't have any evidence that this will make a difference
* Dev convenience: Ignore files generated by editors
* Typo fix
* Was getting a 2000ms timeout on the 'before' hook I believe
* Change certain tests to go directly to their localhost page instead of loading the html content programmatically in order to avoid triggering an incorrect BackCompat mode (incorrect in that the html content has a correct doctype)
* Add test based on motivating site that had images lined up in a square which were all different sizes; very old style percentage width/height attributes were doing the right thing in quirksmode, which is what we are testing for here
* Fixup rrweb test html to include a valid doctype and avoid BackCompat to ensure we're not accidentally testing against quirks modes. I didn't find an elegant way of avoiding the `BackCompat` when adding a minimal iframe, so some BackCompat has slipped in here, I don't think there's much harm
- encountered inconsistent html with multiple <option> elements with the same value attribute
- due to check `attributes.selected = (n as HTMLOptionElement).selected;` the extra ones were being stored as `selected: false`; furthermore, the `false` value was being ignored upon replay, and so the last of the extra ones was being chosen as the 'selected' option
