Another reason to check is when developing in JavaScript you sometimes accidentally have duplicate ID's without realising it. W3C check highlights this immediately saving you time debugging. Also depending on your framework, some PHP errors are hidden inside HTML but not visible in the browser. Again W3C check will reveal that too. The

I have developed a JavaScript solution that shows a small button at the bottom of your page that calls W3C API and when clicked. The button either goes green or red with an error count. The error report is stored inside the browser console.

Just incluse the JavaScript file while running on your development platform. Willing to share if anyone is interested.