Question

I’m confused with HTML and XHTML specifications, which of these should I chose from and why.

For example, a web application written in PHP that searches an image database and shows matches in a dynamic way (something like an infinite scroll) should use what kind of doctype?

Answer 1

Your choice of doctype has nothing to do with the language or applications you will be using. It’s purpose is to indicate to the browser the document’s type definition, that is, the “type” of the tags used in the web page; similar to how “type” is used in programming languages. However, browsers do not look this up on the internet cause it’s built in to the browser program and, nowadays, it’s only used to indicate whether the browser should be in “standards mode” or “quirks mode”. So the choice is HTML or XHTML.

The new doctype, as shown above, can be used with either HTML or XHTML. If you were to use one of the previous doctypes, you would have to use the HTML 4.01 version for HTML, or the XHTML version if you are serving your markup as XHTML (and not HTML).

Far too often we see people using the XHTML doctype but serving the pages as HTML. Browsers treat that markup as “tag soup” or “broken HTML” because XHTML is an application of XML.

New web pages have no use for the “transitional” version as that is for web pages that are “transitioning” from using older, deprecated markup to current standards compliance. It allows current pages to validate even though the elements contained are deprecated.