If you follow the standards, any layout errors will be much easier to debug. When you ignore the standards, the browser can render your page however it feels like, and changing one character can completely change how your page is rendered. Debugging one browser is hard, but debugging IE 7, IE 6, Firefox, Opera, Safari... and ignoring the standards is just going to make your life difficult.
If you are using jQuery or other DOM manipulation libraries, you can get unexpected and inconsistent results if your markup is not valid.
So, don't waste your time. Make sure you write valid HTML and CSS. It's really really easy, and will save you time.
