Once you install and activate the module, you'll be able to see on the User side the CSS markups.
cssHolmes is a XOOPS module with diagnostic CSS stylesheet that can highlight potentially invalid, inaccessible or erroneous HTML(5) markup by adding one class.
cssHolmes is useful for checking the quality of your code (up to W3C HTML5 standards), nitpicking over ensuring markup is valid and semantic and accessibility guidelines are met, 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.
It is NOT recommended for live environments.
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.
In the Userside of the cssHolmes module you will see a testsuite: