Here are some common problems with HTML code. Many of these also apply to XHTML.
To check the easy stuff with HTML:
Figure 22.2. Can you see where the problem is? I've misspelled src. I can't tell you how many times I've torn apart a table or some other complicated construction only to find a miserable little typo like this.
Figure 22.3. The corrected version shows the src attribute spelled correctly, plus I've removed the final / that is used only in XHTML, not HTML.