| 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 :mimetype:`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 behavior 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.) |
|
|
| .. tabularcolumns:: |p{9cm}|p{2cm}|p{2cm}|p{2cm}| |
|
|
| + |
| | | HTML4.01 | XHTML1.1 | HTML5 | |
| +=========================================+==========+==========+==========+ |
| | ``<tag/value/`` == ``<tag>value</tag>`` | |Y| [1]_ | |N| | |N| | |
| + |
| | ``<br/>`` supported | |N| | |Y| | |Y| [2]_ | |
| + |
| | ``<script/>`` supported | |N| | |Y| | |N| | |
| + |
| | should be served as `text/html` | |Y| | |N| [3]_ | |Y| | |
| + |
| | should be served as | |N| | |Y| | |N| | |
| | `application/xhtml+xml` | | | | |
| + |
| | strict error handling | |N| | |Y| | |N| | |
| + |
| | inline SVG | |N| | |Y| | |Y| | |
| + |
| | inline MathML | |N| | |Y| | |Y| | |
| + |
| | ``<video>`` tag | |N| | |N| | |Y| | |
| + |
| | ``<audio>`` tag | |N| | |N| | |Y| | |
| + |
| | New semantic tags like ``<article>`` | |N| | |N| | |Y| | |
| + |
|
|
| .. [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. |
|
|
| .. |Y| image:: _static/yes.png |
| :alt: Yes |
| .. |N| image:: _static/no.png |
| :alt: No |
|
|
| 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: |
| |
| .. sourcecode:: html |
| |
| <!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 book, `Dive Into HTML5`_. |
| Not all of them are supported in browsers yet, however, so use caution. |
|
|
| .. _Dive Into HTML5: https://diveintohtml5.info/ |
|
|
| 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 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. |
|
|
