fixContainer produces a tree with invalid content

[mossymaker]

Steps

1. copy the text from this Pen
2. paste the text into Squire
3. Use Squire's getHTML method; save the resulting DOMString somewhere
4. Use setHTML to restore the DOMString into Squire

Expected: content appears as it did when pasted
Actual: content has additional spacing

Notes

The spacing is caused by giving invalid markup to the editor's root element innerHTML. The invalid markup is introduced when fixContainer wraps inline content with a <div>, but without checking whether the wrapper element is a permitted child of the enclosing element (e.g. <div> is not a permitted in a <span>).

When setHTML gives the content to the root element's innerHTML, the browser doesn't create the same tree from when innerHTML was read.

I have a fix for this...brb.