Document non-standard limitations imposed by hashspace

[jakub-g]

• attributes have to be double-quoted, apostrophes are not allowed (this is pegjs limitation AFAIU)
• <template> and soon <script> tags and their closings have to be placed at the beginning of the line (this is imposed by ourselves to facilitate the parsing)
• closing tags are mandatory (vs html that allows some of them to be optional)

feel free to edit this issue with the other limitations

[benouat]

We should allow apostrophes for attributes. The only limitation we should have is to prevent no-quote at all.

<div attrib="value" other-attrib='value'></div> should be valid to me.