[DOC beta] Fix default value of RenderBuffer's props
Merge pull request #4928 from tricknotes/render-buffer-doc

[DOC beta] Fix default value of RenderBuffer's props
[BUGFIX beta] Normalize HTML string for IE8

This fixes are specified for IE8.

* Force HTML tag to lowercase.
* Add several `class` to be quoted
* Trim newline at the beggining of HTML tag