This is actually a serious suggestion, again with excellent price-performance
ratio. Just edit your sources in a plain text editor with basic support for
line numbers and indentation, and load it into IE5 to get it validated, and to have
reported back the errors in your document by line number and position.
Most HTML tools we have basic XML/XSL support like this one, where you can
load and save files with .xml and .xsl file extensons, and have the generic
tag and line number support, without any special features.