Do you actually have a solid understanding of HTML, CSS, JavaScript, and JQuery? In terms of depth, not just breadth?
I'm going to rattle off a few questions that come up as part of the daily job duties of a Google Frontend SWE, and may frequently show up as phone screen or "warmup" interview questions (the actual coding problems are under NDA and tend to get banned when they appear in public, but these are basic enough that I don't think my coworkers will mind):
1.) Why might you choose an iFrame over XHR for communicating with the server?
2.) What are some ways that you can get JavaScript onto a third-party website that can then communicate with your server and display some data on their website?
3.) Can you write a DOM traversal without using JQuery or other third-party libraries? Why would you want to?
4.) Which DOM API calls are slow and what alternatives do you have for them?
5.) Say that you have too many CSS rules for IE and need to come up with a quick fix that doesn't involve refactoring your CSS into something more sane or reducing the number of CSS rules. How might you do this?
6.) Which tags are self-closing in HTML5? Which tags may be omitted entirely? You can look them up, but which site would you go to?
7.) How do you make an AJAX app back-button aware in different browsers, and how will HTML5 help this?
8.) How might you track that a user has clicked on a link, and again, how might HTML5 help this?
9.) Do you know about Canvas, localStorage/sessionStorage, the <video> tag, WebGL, and various other HTML5 goodies?
10.) How do you write custom attributes on HTML tags in HTML5?
All of these come directly from actual problems we've faced at Google, BTW. If you know 'em, great - I'd pick up one of Python or Ruby and learn how the server works (and we're hiring, BTW). If you don't, I'd go deeper before going broader.
All of the questions, BTW, boil down to one meta-question: do you think about the whole user experience (including latency, stability, integration with third-party sites, and other UX conveniences), or only about putting pixels on the page?
(Edit: Also, Patrick's advice is great. My advice is mostly training you how to make yourself more valuable as an employee. His is training you how to make yourself more valuable as an entrepreneur. The two skillsets are not mutually exclusive - hell, I became an employee because I needed to learn some skills so I could become a better entrepreneur - but they optimize for different paths.)