Make html files of editor and viewer HTML5 validated

Check done with http://validator.w3.org/check

Open questions:
* Not sure what to put into `<title> </title>` for the viewer HTML files, any proposals? At least for validation the tag needs to be there and non-empty
* `<option id="customScaleOption" value="custom"> </option>` needs to have some non-empty content as well to be validated. Actual content is set by the code, but might be good to still have the original html validated. Any other sane default content there then one empty space?