First of all, according to HTML and XHTML standards, each HTML document requires a DOCTYPE (stands for document type declaration). The "DOCTYPE" appears at the top of each HTML document and tells a validator which version of HTML to use in checking the document's syntax. A DOCTYPE is a key component of standards compliant code, and markup and CSS won’t validate without them. If you are using XHTML to code a web page, you have a choice between using a Strict or a Transitional DOCTYPE. The Strict DOCTYPE means that your XHTML has to be completely standards-compliant. The Transitional DOCTYPE allows you to use some elements and attributes that were common in older HTML markup but that do not adhere to current standards.
To summarize (and as its name suggests), transitional DOCTYPES are meant for developers making the transition from older markup to modern ways of building code. On the other side, strict DOCTYPES are actually the default - the way HTML 4.01 and XHTML 1.0 were intended to be utilized. A Strict DOCTYPE also ensures that browsers use their strictest, most standards compliant rendering modes.
TBH Creative uses Strict DOCTYPE for its web pages for these reasons. Using an incomplete or outdated DOCTYPE—or no DOCTYPE at all—throws these same browsers into “Quirks” mode, where the browser assumes you have written old-fashioned, invalid markup and code per the depressing industry norms of the late 1990s. For developers making the switch from Transitional to Strict, below are links to lists of elements allowed with a Transitional but not Strict DOCTYPE: