A webpage consists of text. When you display a webpage in your browser, it usually has a nice layout with sections, columns, and paragraphs. Your browser knows how to layout the webpage because it follows layout instructions embedded in the text. The instructions are called HTML (Hypertext Markup Language). The actual instructions themselves are called "HTML tags". The tags themselves are text, and the text of each tag is identified by a leading < (less-than) character, and a following > (greater-than) character. A specific HTML tag to create a table is shown below.
The first HTML was used in 1989 by the physicist Tim Berners-Lee working at the CERN particle physics laboratory in Geneva Switzerland. It was used to share documents. HTML was derived from a formatting language called SGML (Standard Generalized Markup Language) which was created by the IETF (Internet Engineering Task Force) in 1993.
HTML is sometimes called a markup language because it's used to "markup" text and data to add instructions for how it should be laid out. Initially HTML started out with tags for 18 elements. But as later versions were specified the number of tags grew. Then along came XML (Extensible Markup Language).
Although XML looks similar to HTML, XML actually tells you nothing about the layout of the document. Instead it describes the data within the XML tags. I know that I promised to focus on just HTML, but let me explain why I mention XML. XML has strict rules that were eventually applied to HTML creating what is called XHTML.
Lets compare the tag for a button in HTML with the tag for a button in XHTML.
<input type="button" value="Calculate">
Note in the XHTML example, there's an opening tag <button> and a closing tag </button>, while the HTML example has just an opening tag. The closing tag is specified using a / (slash) after the tag's < (less-than) character. In XHTML it's also legal to use just one tag and place the / just before the > (greater-than) character as shown below.
<input type="button" value="Calculate" />
This form is sometimes preferred because it's understood by older non-XHTML compliant browsers.
Another strict rule of XHTML is that tags must be nested in reverse order of how they were declared. In other words, in HTML the following code was acceptable.
However in XHTML the tags must be nested like this:
Or like this:
Imposing these rules makes XHTML display more reliably and makes its it easier to understand, and I believe closing tags and reverse order tag nesting is the future of HTML (XHTML). It would be a disservice for me to teach you bad old unstructured HTML, so although the title of this eBook is Just HTML, you'll actually be learning just XHTML.
Another version of HTML (HTML5) designed primarily for publishing for eBook readers and mobile devices has been released. However, not all browsers have implemented the entire HTML5 specification, and before they do, it looks like the XHTML5 specification will be released. So near the end of this eBook, I introduce some features of HTML5.
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