What do you mean when you mean errors? Like missing tags, missing links, or much worse errors? I love using chrome and doing the right click and inspect and checking everything on there
Chrome/Firefox and even Safari (to a less extent) are good for diagnosing issues.
Also the https://validator.w3.org/ can be helpful for checking your HTML is complete and standards compliant - though sometimes the problem is not near the issue it caused but it gives you a clue of what to look for.
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/).