Valid XHTML

HTML is a mark-up language used to create websites, and is independent of platforms.

"HTML is not a programming language, since HTML does not allow to, say, control a robot, set a variable or programme a game." (Quelle)

The display of invalid or incorrect documents depends on the software reading them (browser, roboter, screenreader, etc; see above). However, the display will vary greatly depending on the software, and this can lead to errors, often without the knowledge of the website manager, since it is impossible to test the website against all updates of all the software.

Non validated HTML source text can cause high costs, especially if it causes the software to look for an error or if the functionality and therefore the user experience is impacted.  


Further information:
The validity of the HTML source text can be checked with a so-called 'validator'.

Examples:

• http://validator.w3.org/ (a page of a web site)
• Qualidator SiteAnalyzer (all pages of a web site)

Further information
http://blog.antikoerperchen.de/beitrag/28/validierung-wozu-warum-valider-html-code-wichtig-ist.html

Acronyms: 
HTML: HyperText Markup Language

Document Type Declaration

HTML is only one of a series of markup languages, though it is the best known one. HTML is relatively old and has seen various, rather different variations. The Document Type Declaration indicates which markup language and which version of it has been used. A sorting software, e.g. a web browser, can understand this indication.

Source and further information: http://de.selfhtml.org/html/allgemein/grundgeruest.htm.