All markup languages have three aspects: theory, application and philosophy.
Most web developers do not concern themselves with the theory of markup languages, e.g., HTML, XHTML and XML. That is for those who write the specifications and UAs, or User Agents. UAs make things work. Most web developers are not interested in how things works as long as it works.
Most web developers, however, are concerned with the practical application of markup languages in websites they construct. Specification requirements are easier to understand.
HTML 5 isn't any different. It has its theory but that is not what this article is about. (The theory of HTML 5 can be saved for a later day.) The following addresses the application of HTML 5 by web developers while an attempt is made to understand those reasons which make HTML 5 different.
Three fundamental considerations are made by web developers. They are:
1. Document Type Declaration
The W3C DTD, or Document Type Definition, specifies either “Standards Mode” or “Quirks Mode” for UAs parsing Cascading Style Sheets (CSS). (“Standards Mode” is the default for XHTML [except when an XML declaration has been included above the DTD which will then trigger “Quirks Mode”].) Web developers who have chosen HTML 4.01 use DTDs which trigger “Standards Mode”. HTML5 specifies a Document Type Declaration, i.e.,
<!DOCTYPE html>. which triggers “Standards Mode”. (This DocType was not invented by The WHAT WG; it existed previously.) Further, DTDs are unnecessary for elements and attributes. All elements and attributes are recognized by UAs, e.g., browsers, with this DocType.
2. MIME Type
HTML 4.01 is primarily sent as “
text/html”. XHTML 1.0 is primarily sent as “
text/html”. Web Applications 1.0, 1.4.1 - HTML vs XHTML states that all documents sent as “
text/html” are HTML5.
XHTML introduced the concept of “well-formedness”. (See XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition), Appendix C.) “Well-formedness” was simple. However, these days, “well-formedness” has come to include all of the requirements set forth in Appendix C and Section 4. Differences with HTML 4, too. It is one of the cardinal principles of web standards. “Well-formed” sites define web standards.
Most web developers want their CSS displayed in a “standards-compliant mode”; most web developers will continue sending documents as “
text/html”; and, most will not veer from writing “well-formed” code.
It – Then - seems that all one needs to do for using HTML 5 is:
- Replace the W3C Document Type Definition with
- Continue sending documents as “
- Do not alter “well-formed” source code.
Nevertheless, all website pages remain (X)HTML but with a different DocType.
So, theory aside, the application of HTML 5 isn’t any different from common, existing practices practiced by authors who use web standards.
It’s philosophically different. That's all.