This can force your readers to scroll horizontally to read your content, or can cause printed versions to be cropped. For readers whose displays are narrower than the author anticipated, fixed-width tables cause the same problems as other oversized tables. For readers whose displays are wider than the author anticipated, fixed-width tables cause extremely wide pargins, wasting much of the display area. For readers who need larger fonts, fixed-width tables can cause the content to be displayed in short choppy lines of only a few words each. Many browsers are especially sensitive to invalid syntax when tables are involved.
Correct syntax is especially critical.
Even with correct syntax, nested tables may not display correctly in Netscape. Some browsers ignore tables, or can be configured to ignore tables. These browsers will ignore any layout you've created with tables. Also, search engines ignore tables. Some search engines use the text at the beginning of a document to summarize it when it appears in search results, and some index only the first n bytes of a document.