Uses updated Holmes CSS v1.0.46 released on Nov 22 2014.
The extension will display either an error (red outline), a warning (yellow outline), or a deprecated style (dark grey outline) for flags such as:
Missing required attributes on tags, such as alt tags on images (lots of these)
Deprecated and Non-W3C Elements - see W3C.org's article on obsolete tags
Non-W3C Attributes - as above, just the most important ones since there are MANY
Hovering over most elements will cause a description to be displayed in the top left, or top right corner of the screen.
Why should I use it?
holmes.css is useful for checking the quality of your code (up to W3C HTML5 standards), nitpicking over ensuring markup is valid and semantic, and when you are tasked to fix up and debug an old, OLD website. It has a simple implementation and a mostly unobtrusive effect on your page. Not recommended for live enviroments.
Remember too that these are just guidelines: if something is flagged but you can't change it for a good reason, don't worry about it :) Also use a validator if you want to be 100% sure.