It took me ages to find these tools for testing a website. If you write your .html and .css files in Notepad (or the more useful Notepad++) rather than using Wordpress or similar, these tools will check the coding, page loading speed, security and many other aspects to get your website working well. If you view the coding of my web pages, feel free to copy any useful bits. Note that the links below open in a new tab or window.
Online HTML Editor