Build a Chrome Extension that detects some specific HTML in page.

I want a Chrome Extension that detects some specific HTML in page. For example, it could be a variable name in a <script> tag, or anything basically, as long as it's part of the HTML. The text to detect should be configurable. In can be on a single line, something pretty basic. On every page load, it should try to detect that text, on any site. When it does detect it, it shows a pop up or a notification to tell me something like: "Code detected". Any questions?