structural:expansion, stylistic:tone
Transitional versus strict

HTML 4 defined three different versions of the language: Strict, Transitional (once called Loose), and Frameset. The Strict version is intended for new documents and is considered best practice, while the Transitional and Frameset versions were developed to make it easier to transition documents that conformed to older HTML specifications or did not conform to any specification to a version of HTML 4. The Transitional and Frameset versions allow for presentational markup, which is omitted in the Strict version. Instead, cascading style sheets are encouraged to improve the presentation of HTML documents. Because XHTML 1 only defines an XML syntax for the language defined by HTML 4, the same differences apply to XHTML 1 as well.

The Transitional version allows the following parts of the vocabulary, which are not included in the Strict version:

The Frameset version includes everything in the Transitional version, as well as the frameset element (used instead of body) and the frame element.