Not a mojoPortal bug as such, except that it affects both the provided editors... they both like to use border, cellspacing and cellpadding attributes for tables instead of CSS, meaning that with an HTML5 skin pages will not pass W3C validation (without significant extra knowledge and editing by the user to achieve the same effect with CSS).

In TinyMCE this cannot be done without editing the source html, though at least it doesn't add these attributes until you specify values for border, spacing and padding in the table properties. CKEditor emits these attributes as defaults unless you positively clear them, but you can edit the CSS style in the UI.

Is there any way to make these editors more HTML5 compliant?

I've seen this for TinyMCE: http://www.tinymce.com/wiki.php/Configuration:schema and had a nose around the mojo code... finding that the tinymce.init({... script is built in code, and already has script.Append(",schema:'html5'");   Perhaps this would need to be "html5-strict" to prevent it using the obsolete attributes? Perhaps that has downsides? Could/should this be configurable instead of baked in?

I've not found anything for CKEditor, but probably not looked in the right places!