Using CSS to improve the formatting and make the resume more readable would probably be effective at demonstrating a subset of your skills on a small scale. If, however, you went over the top with too much formatting or other bling, the presentation could become distracting from the actual content. You want to showcase your skills and abilities, with a tastefully chosen subset of your abilities that are relevant to the presentation of the document.
Keep in mind, your interviewer(s) could also choose to look at your source-code, given the nature of the position you are applying for, so you would want to demonstrate the sort of clean-well-formatted code you would be producing for them should they hire you. You might even want to add some embedded comments in your source-code, if appropriate, explaining design decisions you made, etc. For example, you might explain why you chose to put your styles in a style block in the header of the page, rather than in a separate file vs attached to html entities with a style attribute. Obviously, the choice you would make for a file that might be passed around as an email attachment or possibly even read offline, might be different than what you would choose for a live-website. Having everything in one file, so when a less computer-savvy team member reads the resume and double clicks the attachment rather than saving all attachments and then opening the html file, you don't risk the formatting being missing.
But all in all, don't lose sight of the fact that the purpose of this document is a resume, having well-written content selling your skills and past experience, all things that you feel confident discussing on your interviews may be even more important than the specific formatting you chose to use.