Table attributes produced by CKEditor are obsolete in HTML 5

The table dialog of CKEditor has options for specifying cellpadding, cellspacing, border, width, height and alignment:

 

 
These attributes are obsolete in HTML 5 and must not be used anymore. The attributes are persisted into XWiki markup so even if we should remove them from the dialog, existing content will continue to have them and users will expect to be able to use them. We will thus probably need to change CKEditor to produce CSS instead if we want to keep these inputs.

We can disable these attributes using disallowed content but for existing content this will probably just remove them without replacement. I'm also not sure how easy it is to let these inputs control equivalent CSS properties instead. I'm thus not sure if this should only be implemented in CKEditor or if we shouldn't also change the HTML5 renderer to transform these attributes into CSS such that the formatting is not lost in existing content if we configure CKEditor to strip these attributes.

This is independent of CKEditor using xhtml/1.0 or html/5.0 as the result is rendered as HTML 5 in both cases. Even if we changed the HTML 5 renderer to transform these attributes, CKEditor would need to be adapted as the values are then not found in the content anymore as expected when switching to HTML 5.