** This plugin is no longer necessary with the release of Sigil v0.9.5. The functionality it provided is now included within Sigil itself. There are settings under Sigil's preferences (the General section) for what CSS level you want the "Validate Stylesheets with W3C" tool to use. **
I'm leaving this thread here for educational purposes (and for users of older versions of Sigil).
CSS3Validator
A temporary, stop-gap validation plugin to allow the CSS from EPUBs to be validated (the online W3C validator) against CSS level 3 (especially handy for EPUB3s).
This plugin should become unnecessary when Sigil 0.9.5 is released -- since it will include the option to choose the CSS Level you wish to validate your EPUB2's stylesheets against.
Works much the same way as Sigil's built-in "Validate Stylesheets With W3C" tool. It writes temporary files that are then launched in the system's default web browser (just like the Sigil tool).
There are three configurable options in the plugin's preference json file (available after the plugin runs once):
css_level String value (defaults to "css3"): choices are "css2", "css21" or "css3". Determines the CSSlevel you want the validator to use.
delayAn integer (defaults to 3): the number of seconds the webpage will delay before automatically posting its data to the W3C website. This is only relevant if your browser has javascript disabled (you'll have to click the 'check' button manually in case). If you have javascript enabled, there's no reason you can't set the delay to 0 seconds if you want to speed things up.
delete_tmp_folderA boolean value (defaults to true): the plugin will attempt to clean up after itself. On older, slower machines, this may have the undesirable consequence of deleting the temporary files before the web browser has had a chance to load them. If you find that happening, change this value to false. Then no attempt will be made to delete the temporary files. You'll have to clean your temp files yourself occasionally if that's the case. All temporary files will be in a folder with the prefix "CSSValidator-" if you're interested.