Gregor J. Rothfuss, previously of Endoxon but now a Googler, has just given Feedvalidator the ability to validate KML, in addition to RSS and Atom. Here’s what it looks like when I submit one of my own KML files — it is found to be somewhat lacking:-):

This newfound ability of Feed Validator is not documented anywhere on the site yet — it’s been included in the nightly builds since May 30. Just remember to feed it KML, not KMZ/.It even does KMZ!:-) Practically, Google Earth is a lot more lenient than Feed Validator, though of course the whole point of validators is that they be unforgiving:-)

You can download and run Feedvalidator’s Python code for off-line testing. There are folders in place ready to hold KML 2.2 validation rules, though they do not appear to be fleshed out yet.