The W3C validator that haydn! mentions has always been the standard. If you are looking for malformed HTML then also look at the documentation for you editor of choice to see if it has a syntax checker. For example, BBEdit has a nice checker and even editors that don't have a dedicated feature will surely at least indirectly indicate problems with structure in the syntax colorization.
For Javascript you should be using a linter like ESLint to check for errors and also can use the Console in any modern browser's developer tools to loor for execution errors.
For LESS or SASS, your compiler should be flagging any errors.
One excellent resource for checking for web development errors of nearly any language other that HTML during compiling, is the amazing CodeKit application (
https://codekitapp.com/).