ID's changed for Principles, defs between ATAG 2 and respec version

[hidde]

eg #principle_a1 changed to #part-a-principle-a.1, which would break linking if we publish (eg from tools like ATAG Report Tool).

[hidde]

If we want to update the content to be mostly Markdown rather than HTML, which I think would be reasonable and make editing easier, that might be a good time to decide on not worrying about keeping ID's same and rely on the auto generated ones instead.

[reidmore-online]

I'm reluctant to introduce markdown only because while I stripped out a great deal of "custom" formatting when converting 2.0 to respec, I imagine we are going to re-introduce a lot of it when we start working towards the new revision. You can intersperse HTML with markdown, but I have found in the past it gets rather finicky and I end up converting back to HTML anyway.