+ +

+

HTML/XHTML FAQ¶

+

The Flask documentation and example applications are using HTML5. You +may notice that in many situations, when end tags are optional they are +not used, so that the HTML is cleaner and faster to load. Because there +is much confusion about HTML and XHTML among developers, this document tries +to answer some of the major questions.

+

History of XHTML¶

+

For a while, it appeared that HTML was about to be replaced by XHTML. +However, barely any websites on the Internet are actual XHTML (which is +HTML processed using XML rules). There are a couple of major reasons +why this is the case. One of them is Internet Explorer’s lack of proper +XHTML support. The XHTML spec states that XHTML must be served with the MIME +type application/xhtml+xml, but Internet Explorer refuses to read files +with that MIME type. +While it is relatively easy to configure Web servers to serve XHTML properly, +few people do. This is likely because properly using XHTML can be quite +painful.

+

One of the most important causes of pain is XML’s draconian (strict and +ruthless) error handling. When an XML parsing error is encountered, +the browser is supposed to show the user an ugly error message, instead +of attempting to recover from the error and display what it can. Most of +the (X)HTML generation on the web is based on non-XML template engines +(such as Jinja, the one used in Flask) which do not protect you from +accidentally creating invalid XHTML. There are XML based template engines, +such as Kid and the popular Genshi, but they often come with a larger +runtime overhead and, are not as straightforward to use because they have +to obey XML rules.

+

The majority of users, however, assumed they were properly using XHTML. +They wrote an XHTML doctype at the top of the document and self-closed all +the necessary tags (<br> becomes <br/> or <br></br> in XHTML). +However, even if the document properly validates as XHTML, what really +determines XHTML/HTML processing in browsers is the MIME type, which as +said before is often not set properly. So the valid XHTML was being treated +as invalid HTML.

+

XHTML also changed the way JavaScript is used. To properly work with XHTML, +programmers have to use the namespaced DOM interface with the XHTML +namespace to query for HTML elements.

+

History of HTML5¶

+

Development of the HTML5 specification was started in 2004 under the name +“Web Applications 1.0” by the Web Hypertext Application Technology Working +Group, or WHATWG (which was formed by the major browser vendors Apple, +Mozilla, and Opera) with the goal of writing a new and improved HTML +specification, based on existing browser behaviour instead of unrealistic +and backwards-incompatible specifications.

+

For example, in HTML4 <title/Hello/ theoretically parses exactly the +same as <title>Hello</title>. However, since people were using +XHTML-like tags along the lines of <link />, browser vendors implemented +the XHTML syntax over the syntax defined by the specification.

+

In 2007, the specification was adopted as the basis of a new HTML +specification under the umbrella of the W3C, known as HTML5. Currently, +it appears that XHTML is losing traction, as the XHTML 2 working group has +been disbanded and HTML5 is being implemented by all major browser vendors.

+

HTML versus XHTML¶

+

The following table gives you a quick overview of features available in +HTML 4.01, XHTML 1.1 and HTML5. (XHTML 1.0 is not included, as it was +superseded by XHTML 1.1 and the barely-used XHTML5.)

+ ++++++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

	HTML4.01	XHTML1.1	HTML5
`<tag/value/` == `<tag>value</tag>`	[1]
`<br/>` supported			[2]
`<script/>` supported
should be served as text/html		[3]
should be served as +application/xhtml+xml
strict error handling
inline SVG
inline MathML
`<video>` tag
`<audio>` tag
New semantic tags like `<article>`

+ + + + + +

[1]	This is an obscure feature inherited from SGML. It is usually not +supported by browsers, for reasons detailed above.

+ + + + + +

[2]	This is for compatibility with server code that generates XHTML for +tags such as `<br>`. It should not be used in new code.

+ + + + + +

[3]	XHTML 1.0 is the last XHTML standard that allows to be served +as text/html for backwards compatibility reasons.

+

What does “strict” mean?¶

+

HTML5 has strictly defined parsing rules, but it also specifies exactly +how a browser should react to parsing errors - unlike XHTML, which simply +states parsing should abort. Some people are confused by apparently +invalid syntax that still generates the expected results (for example, +missing end tags or unquoted attribute values).

+

Some of these work because of the lenient error handling most browsers use +when they encounter a markup error, others are actually specified. The +following constructs are optional in HTML5 by standard, but have to be +supported by browsers:

+

Wrapping the document in an <html> tag
Wrapping header elements in <head> or the body elements in +<body>
Closing the <p>, <li>, <dt>, <dd>, <tr>, +<td>, <th>, <tbody>, <thead>, or <tfoot> tags.
Quoting attributes, so long as they contain no whitespace or +special characters (like <, >, ', or ").
Requiring boolean attributes to have a value.

+

This means the following page in HTML5 is perfectly valid:

+

<!doctype html>
+<title>Hello HTML5</title>
+<div class=header>
+  <h1>Hello HTML5</h1>
+  <p class=tagline>HTML5 is awesome
+</div>
+<ul class=nav>
+  <li><a href=/index>Index</a>
+  <li><a href=/downloads>Downloads</a>
+  <li><a href=/about>About</a>
+</ul>
+<div class=body>
+  <h2>HTML5 is probably the future</h2>
+  <p>
+    There might be some other things around but in terms of
+    browser vendor support, HTML5 is hard to beat.
+  <dl>
+    <dt>Key 1
+    <dd>Value 1
+    <dt>Key 2
+    <dd>Value 2
+  </dl>
+</div>
+

+

+

New technologies in HTML5¶

+

HTML5 adds many new features that make Web applications easier to write +and to use.

+

The <audio> and <video> tags provide a way to embed audio and +video without complicated add-ons like QuickTime or Flash.
Semantic elements like <article>, <header>, <nav>, and +<time> that make content easier to understand.
The <canvas> tag, which supports a powerful drawing API, reducing +the need for server-generated images to present data graphically.
New form control types like <input type="date"> that allow user +agents to make entering and validating values easier.
Advanced JavaScript APIs like Web Storage, Web Workers, Web Sockets, +geolocation, and offline applications.

+

Many other features have been added, as well. A good guide to new features +in HTML5 is Mark Pilgrim’s soon-to-be-published book, Dive Into HTML5. +Not all of them are supported in browsers yet, however, so use caution.

+

What should be used?¶

+

Currently, the answer is HTML5. There are very few reasons to use XHTML +considering the latest developments in Web browsers. To summarize the +reasons given above:

+

Internet Explorer (which, sadly, currently leads in market share) +has poor support for XHTML.
Many JavaScript libraries also do not support XHTML, due to the more +complicated namespacing API it requires.
HTML5 adds several new features, including semantic tags and the +long-awaited <audio> and <video> tags.
It has the support of most browser vendors behind it.
It is much easier to write, and more compact.

+

For most applications, it is undoubtedly better to use HTML5 than XHTML.

+

+ + +

HTML/XHTML FAQ¶

History of XHTML¶

History of HTML5¶

HTML versus XHTML¶

What does “strict” mean?¶

New technologies in HTML5¶

What should be used?¶

Table Of Contents

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HTML/XHTML FAQ¶

History of XHTML¶

History of HTML5¶

HTML versus XHTML¶

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New technologies in HTML5¶

What should be used?¶

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