Validation is an integral part of web development. HTML and CSS validation ensure that the document is structurally and semantically correct and provide a reliability guarantee that the page will be displayed correctly, both now and in the future.
In this recipe, we will look at how the Web Developer add-on provides easy options for validating both HTML and CSS using the W3C validator at http://validator.w3.org.
It is assumed that the Web Developer add-on is installed and enabled in Firefox.
To validate the HTML of a page currently being viewed in Firefox, click on the Tools menu of the Web Developer toolbar and select Validate HTML as displayed in the following screenshot: ...