Definitely add the base href tag.
Those error messages from the validator are pretty clear. Your strong tag is missing a closing tag and you have spaces in your paths that should be removed (and the files and directories renamed to contain no spaces in their names), or replace the spaces with %20 in the link. The first is probably a better choice.
We can help you with MODX, but we generally don't have time to teach people HTML or CSS here.

There are plenty of good HTML and CSS tutorials on the web and you can Google any errors you don't know how to fix to get more information.
If you're pasting code in at the validator, do a "view source" on the page and paste that. Then the MODX tags won't cause errors.