There are fixed rules that define which tags may be used in an HTML document, where they can be placed. As your documents get larger it can be difficult to be sure that everything is correct. There are automated tools that will check your HTML for you. These tools are known as validators. Several validators are free to use, including
Try uploading the index.html or index.htm file you created in the previous section to one of the validators listed above. Alternately both validators have options that let you enter HTML directly so you could cut and paste the example from this page into the validator.
There is also an HTML-validating Firefox extension that can validate HTML using either HTML Tidy or the SGML Parser (what the w3 validator is based on). It is available here for all platforms.
It is good practice to validate each HTML document you create. Note that many visual design tools will let you create invalid webpages so it is important to check pages produced in these packages as well.
If the HTML document is valid it means that the webpage will display exactly as you programmed it to on stable W3C compliant browsers. In the case of text browsers such as Lynx, the text will format correctly so that it can be read easily by the consumer. Knowing HTML also means that you can edit the pages created using WYSIWYG programs manually, as these will without fail throw in unnecessary coding which clogs up and slows down the loading of your page.