By Jennifer Kyrnin

An "ML" is a markup language. There are many different markup languages these days. Of course, this site focuses on HTML and how it is used, but there are lots of new (and old) MLs that you should be aware of if you are involved in Web Development, such as: HTML, XML, and XHTML.

What is a Markup Language?
When you format text to be printed (or displayed on a computer or TV), you need to distiguish between the text itself and the instructions for printing the text. The markup is the instructions for the text.

Markup can also indicate information about the text. For example, many students in school will highlight certain phrases in their text books. This indicates that the highlighted text is more important than the surrounding text. The highlight color is markup.

HTML
Hypertext markup language is the language of the Web. All Web pages are written in HTML. HTML defines the way that images, multimedia, and text are displayed in Web browsers. It includes tags to connect your documents (hypertext) and make your Web documents interactive (forms).

HTML is a defined standard markup language. That standard was developed by the World Wide Web Consortium (W3C). It is based upon SGML (Standard Generalized Markup Language). It is a language that uses tags to define the structure and some style of your text. Tags are defined by the < and > characters.

But HTML is no longer the most current standard for Web development. As HTML was developed it got more and more complicated and the style and content tags combined into one language. Eventually, the W3C decided that there was a need for a separation between the style of a Web page and the content. A tag that defines the content alone, such as <h1>, would remain in HTML while, tags that define style, such as <font>, are deprecated in the latest version of HTML in favor of style sheets.

XML
The eXtensible Markup Language is the language that the new version of HTML is based on. XML is also based off of SGML. It is less strict than SGML, but provides the extensibility to create various different languages as they relate to your business. XML is a language for writing markup languages. For example, if you are working on genealogy, you might create tags using XML to define the <father>, <mother>, <daughter>, and <son> in your documents. There are also several standardized languages already created with XML: MathML for defining mathematics, SMIL for working with multimedia, and XHTML.

XHTML
XHTML is HTML 4.0 redefined to meet the XML standard. There aren't a lot of major differences between HTML and XHTML:

• XHTML is written in lower case. While HTML tags can be written in UPPER case, MiXeD case, or lower case, to be correct, XHTML tags must be all lower case.
• All XHTML tags must have an end tag. Tags with only one tag, such as <hr> and <img> need a closing slash (/) at the end of the tag: <hr />
• All attributes must be quoted in XHTML. Many editors remove the quotes around attributes, but they are required for correct XHTML.
• XHTML requires that tags are nested correctly. If you open a bold (<b>) tag and then an italics (<i>) tag, you must close the italics tag (</i>) before you close the bold (</b>).
• XHTML attributes must have a name and a value. Attributes that are stand-alone in HTML must be declared with values as well, for example, the <hr> attribute noshade would be written noshade="noshade".

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