XHTML (eXtensible HyperText Markup Language) is a text-based formatting system that allows an author to designate the structure and importance of pieces of a web page. It is a standard for representing the structure and content of a web page.

Its predecessor HTML not only represented the content and structure, but also included aspects such as the presentation of the information on the screen. However, attempting to change the design and presentation of the information using XHTML is now frowned upon and all page styling should now be done using a technique/standard called Cascading Style Sheets (CSS)

CSS (Cascading StyleSheets) is the language used to style and position elements in a webpage. The advantage of separating content and structure from style and layout is that if, for example, we have a separate CSS file (i.e. the CSS is not embedded into the XHTML), we can easily apply a standard style across a whole website, without duplicating the CSS over and over again (which would inevitably lead to omissions, errors, and maintenance problems).