Besides SEO and the importance of contents, a website meets technical requirements that need to be fullfilled in order to have a valid and high quality website.
This page is a collection of free tools, that help webmasters to understand what typical proplems can be and that provide some analyzing tool for checking a website.
W3C Markup Validation Service
With this tool you can check, if your HTML or XHTML Code is valid. The Tool automatically analyses your Web-URL or direct upload. It gives hints where Problems or Errors were found and provides possible solutions.
Go to: https://validator.w3.org
A very simple, powerful and effective tool to tackle HTML Errors. Also ideal for HTML beginners to check for HTML issues and learn from the results.
W3C CSS Validation Service
Just as easy and effetive as the Markup Service this tool checks the validity of you CSS. Provide an URI or direct upload and the tool analyses it for you:
Simple and effective, also ideal for CSS beginners to learn from the results.
Go to: http://www.jslint.com
Options for validating make it easier to track errors.
PHP syntax checker
Tracking PHP syntax errors can be an exhausting task as the error output is often ambigous.
This tool can help you to find syntax errors faster:
Go to: http://phpcodechecker.com
This tool tests the usabilty, accessibility and SEO of your site:
Go to: http://www.qualidator.com/