Once you have that knowledge, you can then explain it to companies that have not yet seen the light and have rolled their own - probably inferior - framework.
To be clear, we're talking about three distinct technologies here:
- A DOM abstraction library, like jQuery, Prototype, or even Dojo and Ext JS (on the complex end). These libraries wrap the DOM API in a better syntax, smoothing over the browser differences and bugs.
So, my advice would be:
- Learn jQuery.
- Learn about the DOM API, and how it interacts with the in-memory DOM of the browser, but don't dive too deep in browser differences. Just learn the basic architecture of what happens "underneath" libraries like jQuery. Reading the jQuery source may help.
This way you'll understand what the technologies can do, how they interact, and how to use them effectively, which will get you the jobs you want.