Holmes is an HTML testing tool. Holmes tests HTML for invalid, inaccessible or erroneous HTML(5) markup. It highlights errors with red outlines, warnings with yellow outlines and deprecated elements with dark gray outlines. Holmes will highlight missing required attributes on tags, improvable markup, deprecated and non-W3C elements and attributes, and other semantic, validation, and accessibility errors.
To use Holmes — The Markup Detective you just need to grab the holmes.css file, link to it in your HTML document’s Head section as you would with any other stylesheet, then add the class “holmes-debug” to your HTML’s body tag. View the page in a browser and all warnings, errors and depreciated elements will be displayed with red, yellow and dark gray outlines.