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9.4 KiB
207 lines
9.4 KiB
HTML/XHTML FAQ |
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============== |
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The Flask documentation and example applications are using HTML5. You |
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may notice that in many situations, when end tags are optional they are |
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not used, so that the HTML is cleaner and faster to load. Because there |
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is much confusion about HTML and XHTML among developers, this document tries |
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to answer some of the major questions. |
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History of XHTML |
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---------------- |
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For a while, it appeared that HTML was about to be replaced by XHTML. |
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However, barely any websites on the Internet are actual XHTML (which is |
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HTML processed using XML rules). There are a couple of major reasons |
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why this is the case. One of them is Internet Explorer's lack of proper |
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XHTML support. The XHTML spec states that XHTML must be served with the MIME |
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type `application/xhtml+xml`, but Internet Explorer refuses to read files |
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with that MIME type. |
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While it is relatively easy to configure Web servers to serve XHTML properly, |
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few people do. This is likely because properly using XHTML can be quite |
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painful. |
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One of the most important causes of pain is XML's draconian (strict and |
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ruthless) error handling. When an XML parsing error is encountered, |
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the browser is supposed to show the user an ugly error message, instead |
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of attempting to recover from the error and display what it can. Most of |
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the (X)HTML generation on the web is based on non-XML template engines |
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(such as Jinja, the one used in Flask) which do not protect you from |
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accidentally creating invalid XHTML. There are XML based template engines, |
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such as Kid and the popular Genshi, but they often come with a larger |
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runtime overhead and, are not as straightforward to use because they have |
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to obey XML rules. |
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The majority of users, however, assumed they were properly using XHTML. |
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They wrote an XHTML doctype at the top of the document and self-closed all |
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the necessary tags (``<br>`` becomes ``<br/>`` or ``<br></br>`` in XHTML). |
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However, even if the document properly validates as XHTML, what really |
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determines XHTML/HTML processing in browsers is the MIME type, which as |
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said before is often not set properly. So the valid XHTML was being treated |
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as invalid HTML. |
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XHTML also changed the way JavaScript is used. To properly work with XHTML, |
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programmers have to use the namespaced DOM interface with the XHTML |
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namespace to query for HTML elements. |
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History of HTML5 |
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---------------- |
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Development of the HTML5 specification was started in 2004 under the name |
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"Web Applications 1.0" by the Web Hypertext Application Technology Working |
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Group, or WHATWG (which was formed by the major browser vendors Apple, |
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Mozilla, and Opera) with the goal of writing a new and improved HTML |
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specification, based on existing browser behavior instead of unrealistic |
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and backwards-incompatible specifications. |
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For example, in HTML4 ``<title/Hello/`` theoretically parses exactly the |
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same as ``<title>Hello</title>``. However, since people were using |
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XHTML-like tags along the lines of ``<link />``, browser vendors implemented |
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the XHTML syntax over the syntax defined by the specification. |
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In 2007, the specification was adopted as the basis of a new HTML |
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specification under the umbrella of the W3C, known as HTML5. Currently, |
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it appears that XHTML is losing traction, as the XHTML 2 working group has |
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been disbanded and HTML5 is being implemented by all major browser vendors. |
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HTML versus XHTML |
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----------------- |
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The following table gives you a quick overview of features available in |
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HTML 4.01, XHTML 1.1 and HTML5. (XHTML 1.0 is not included, as it was |
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superseded by XHTML 1.1 and the barely-used XHTML5.) |
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.. tabularcolumns:: |p{9cm}|p{2cm}|p{2cm}|p{2cm}| |
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+-----------------------------------------+----------+----------+----------+ |
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| | HTML4.01 | XHTML1.1 | HTML5 | |
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+=========================================+==========+==========+==========+ |
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| ``<tag/value/`` == ``<tag>value</tag>`` | |Y| [1]_ | |N| | |N| | |
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+-----------------------------------------+----------+----------+----------+ |
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| ``<br/>`` supported | |N| | |Y| | |Y| [2]_ | |
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+-----------------------------------------+----------+----------+----------+ |
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| ``<script/>`` supported | |N| | |Y| | |N| | |
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+-----------------------------------------+----------+----------+----------+ |
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| should be served as `text/html` | |Y| | |N| [3]_ | |Y| | |
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+-----------------------------------------+----------+----------+----------+ |
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| should be served as | |N| | |Y| | |N| | |
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| `application/xhtml+xml` | | | | |
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+-----------------------------------------+----------+----------+----------+ |
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| strict error handling | |N| | |Y| | |N| | |
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+-----------------------------------------+----------+----------+----------+ |
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| inline SVG | |N| | |Y| | |Y| | |
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+-----------------------------------------+----------+----------+----------+ |
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| inline MathML | |N| | |Y| | |Y| | |
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+-----------------------------------------+----------+----------+----------+ |
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| ``<video>`` tag | |N| | |N| | |Y| | |
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+-----------------------------------------+----------+----------+----------+ |
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| ``<audio>`` tag | |N| | |N| | |Y| | |
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+-----------------------------------------+----------+----------+----------+ |
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| New semantic tags like ``<article>`` | |N| | |N| | |Y| | |
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+-----------------------------------------+----------+----------+----------+ |
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.. [1] This is an obscure feature inherited from SGML. It is usually not |
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supported by browsers, for reasons detailed above. |
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.. [2] This is for compatibility with server code that generates XHTML for |
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tags such as ``<br>``. It should not be used in new code. |
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.. [3] XHTML 1.0 is the last XHTML standard that allows to be served |
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as `text/html` for backwards compatibility reasons. |
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.. |Y| image:: _static/yes.png |
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:alt: Yes |
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.. |N| image:: _static/no.png |
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:alt: No |
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What does "strict" mean? |
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------------------------ |
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HTML5 has strictly defined parsing rules, but it also specifies exactly |
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how a browser should react to parsing errors - unlike XHTML, which simply |
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states parsing should abort. Some people are confused by apparently |
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invalid syntax that still generates the expected results (for example, |
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missing end tags or unquoted attribute values). |
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Some of these work because of the lenient error handling most browsers use |
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when they encounter a markup error, others are actually specified. The |
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following constructs are optional in HTML5 by standard, but have to be |
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supported by browsers: |
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- Wrapping the document in an ``<html>`` tag |
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- Wrapping header elements in ``<head>`` or the body elements in |
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``<body>`` |
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- Closing the ``<p>``, ``<li>``, ``<dt>``, ``<dd>``, ``<tr>``, |
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``<td>``, ``<th>``, ``<tbody>``, ``<thead>``, or ``<tfoot>`` tags. |
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- Quoting attributes, so long as they contain no whitespace or |
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special characters (like ``<``, ``>``, ``'``, or ``"``). |
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- Requiring boolean attributes to have a value. |
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This means the following page in HTML5 is perfectly valid: |
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.. sourcecode:: html |
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<!doctype html> |
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<title>Hello HTML5</title> |
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<div class=header> |
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<h1>Hello HTML5</h1> |
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<p class=tagline>HTML5 is awesome |
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</div> |
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<ul class=nav> |
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<li><a href=/index>Index</a> |
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<li><a href=/downloads>Downloads</a> |
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<li><a href=/about>About</a> |
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</ul> |
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<div class=body> |
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<h2>HTML5 is probably the future</h2> |
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<p> |
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There might be some other things around but in terms of |
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browser vendor support, HTML5 is hard to beat. |
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<dl> |
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<dt>Key 1 |
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<dd>Value 1 |
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<dt>Key 2 |
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<dd>Value 2 |
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</dl> |
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</div> |
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New technologies in HTML5 |
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------------------------- |
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HTML5 adds many new features that make Web applications easier to write |
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and to use. |
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- The ``<audio>`` and ``<video>`` tags provide a way to embed audio and |
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video without complicated add-ons like QuickTime or Flash. |
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- Semantic elements like ``<article>``, ``<header>``, ``<nav>``, and |
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``<time>`` that make content easier to understand. |
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- The ``<canvas>`` tag, which supports a powerful drawing API, reducing |
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the need for server-generated images to present data graphically. |
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- New form control types like ``<input type="date">`` that allow user |
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agents to make entering and validating values easier. |
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- Advanced JavaScript APIs like Web Storage, Web Workers, Web Sockets, |
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geolocation, and offline applications. |
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Many other features have been added, as well. A good guide to new features |
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in HTML5 is Mark Pilgrim's soon-to-be-published book, `Dive Into HTML5`_. |
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Not all of them are supported in browsers yet, however, so use caution. |
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.. _Dive Into HTML5: http://diveintohtml5.info/ |
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What should be used? |
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-------------------- |
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Currently, the answer is HTML5. There are very few reasons to use XHTML |
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considering the latest developments in Web browsers. To summarize the |
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reasons given above: |
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- Internet Explorer (which, sadly, currently leads in market share) |
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has poor support for XHTML. |
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- Many JavaScript libraries also do not support XHTML, due to the more |
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complicated namespacing API it requires. |
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- HTML5 adds several new features, including semantic tags and the |
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long-awaited ``<audio>`` and ``<video>`` tags. |
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- It has the support of most browser vendors behind it. |
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- It is much easier to write, and more compact. |
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For most applications, it is undoubtedly better to use HTML5 than XHTML.
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