It’s not hard to use Greek letters and math symbols in (X)HTML, but apparently it’s not common knowledge either. Many pages insert little image files every time they need a special character. Such web pages look a little like ransom notes with letters cut from multiple sources. Sometimes this is necessary but often it can be avoided.
I’ve posted a couple pages on using Greek letters and math symbols in HTML, XML, XHTML, TeX, and Unicode. I included TeX because it’s the lingua franca for math typography, and I included Unicode because the X(HT)ML representation of symbols is closely related to Unicode.
The notes give charts for encoding Greek letters and some of the most common math symbols. They explain how HTML and XHTML differ in this context and also discuss browser compatibility issues.