XHTML Quick Reference

XHTML is basically HTML with a smaller list of allowed tags and stricter syntax - if you know HTML already, it shouldn't take long to master XHTML.

Syntax

All tags and attribute names must be lowercase

However attribute values can be mixed case if they are not from a pre-defined set.
e.g. <input type="text" ... /> but not <input type="Text" ... /> (value 'type' belongs to a predefined set); whereas <a href="http://www.sanger.ac.uk/" title="Wellcome Trust Sanger Institute"> is OK (free text value)
All elements which can have end tags must have end tags

e.g. <p>Paragraph</p><p>More</p> not <p>Paragraph <p>More
All elements which do not have end tags ('empty' tags) must have closing slashes
e.g. <br /> <hr /> <img src="image.gif" width="10" height="10 alt="" />
All paired tags must be nested correctly

e.g. <p>This is some <b>bold text with <i>italics</i> inside</b>.</p>

In addition, there are some tags that cannot contain certain other tags:
- The <a> tag cannot contain another <a> tag.
- The <pre> tag cannot contain <img>, <object>, <big>, <small>, <sub> or <sup> tags.
- The <button> tag cannot contain <input>, <select>, <textarea>, <label>, <button>, <form>, <fieldset>, <iframe> or <isindex> tags.
- The <label> tag cannot contain other <label> tags.
- The <form> tag cannot contain other <form> tags.
Attribute values must always be enclosed in quotes

e.g. <td colspan="2"> not <td colspan=2>
All attributes must have values

e.g. <input type="checkbox" name="choose" checked="checked" /> not <input type="checkbox" name="choose" checked />
Anchors used for jumping between links should have an id attribute as well as a name* and the two attributes should have identical values

<a href="#foo">Go to foo</a> <a id="foo" name="foo">Foo</a> Also, ids (and their equivalent names) must match the pattern [A-Za-z][A-Za-z0-9:_.-]*
All ampersands should be encoded as &

This includes alt text, titles and external URLs. E.g. <a href="http://www.misk.edu/genomics.php?project=cthulu&id=hpl">Interesting Page</a>

For internal links, the Ensembl server has been configured to accept a semicolon in URLs instead: e.g. <a href="/Homo_sapiens/ldview?snp=rs1154266;pop=936">

* This is because the use of name as an anchor is deprecated in XHTML 1.0 (it is reserved for the variable names of form widgets) but id is not recognised as an anchor identifier by Netscape 4.

XHTML Elements and Attributes

There are a number of tags in XHTML Transitional that are deprecated in XHTML Strict and may be removed from later versions of HTML. Some of these tags can easily be replaced by CSS and should be eliminated from the Ensembl source code (if present) - others require more complex changes and can thus be left in for the time being.

Deprecated presentational tags - use CSS instead!

<basefont>	<center>	<s>
<font>	<u>	<strike>
<big>	<b>	<sub>
<small>	<i>	<sup>
<tt>	<listing>	<xmp>

Deprecated structural tags

<dir>

<menu>

<embed>

In practice, <applet> is the only one likely to be seen on the Ensembl website, and replacing it with the preferred <object> tag can be fiddly!

Allowed - but not supported by older browsers

<abbr> and <acronym> don't do much in most browsers and can break older ones, so these are best avoided.

Allowed tags

Just about all XHTML elements can take the attributes id, class, style and title; to save typing these out each time, the shorthand [std] is used.

There are two categories of deprecated attributes: presentational ones that can easily be replaced with CSS, which I have denoted with italics, and other attributes for which there is no straightforward replacement.

Element	Allowed attributes	Deprecated attributes
Principal document elements
html	xmlns, xml:lang, lang	version
head	-	-
body	[std]	background, bgcolor, text, link, alink, vlink
Document head
link	href, type, rel, rev, media	target
style	type (required), media, title	-
meta	name, content, http_equiv
Text
div	[std]	align, nowrap
p	[std]	align
span	[std]	-
br	[std]	clear
pre	[std]	width, align
h1...h6	[std]	align
address	[std]	align
blockquote	[std]	align
q	[std]	-
cite	[std]	align
em	[std]	-
strong	[std]	-
dfn	[std]	align
kbd	[std]	align
Rules, images, hyperlinks and image maps
hr	[std]	align, noshade, size, width
img	[std] + src (required), alt (required), height, width, usemap, longdesc	name, align, border, vspace, hspace
a	[std] + href, shape, coords, type, accesskey, tabindex	name, target
area	[std] + alt (required), href, nohref, shape, coords, accesskey, tabindex	target
map	[std] + name
Lists
ul	[std]	type, compact
ol	[std]	start, type, compact
li	[std]	value, type
dl	[std]	compact
dt	[std]	-
dd	[std]	-
Tables
N.B. Note that whilst cellpadding, cellspacing and border attributes are allowable in tables, a more consistent look can be achieved using CSS.
table	[std] + width, cellpadding, cellspacing, border, summary	align, valign, bgcolor
caption	[std]	align
tr	[std]	align, valign, bgcolor
th and td	[std] + colspan, rowspan	align, valign, bgcolor, width, height, nowrap
col	[std]	align, valign
colgroup	[std]
thead	[std]
tbody	[std]
tfoot	[std]
Forms
form	[std] + action (required), method, enctype	target
fieldset	[std]	-
legend	[std] + accesskey	align
label	[std] + for, accesskey	-
input	[std] + type, name, value, checked, size,, maxlength, src, accesskey, tabindex, alt, usemap, accept, disabled, readonly	align
select	[std] + name, size, multiple, tabindex, disabled	-
option	[std] + value, selected, disabled	-
optgroup	[std] + label, disabled	-
textarea	[std] + rows and cols (required), name, accesskey, tabindex, disabled, readonly	-
button	[std] + name, value, type, accesskey, tabindex, disabled
Executable content
script	type (required), src, charset, defer	language
noscript	-	-
object	[std] + classid, type, data, codebase, archive, codetype, declare, standby	align, width, height, border, hspace, vspace
param	type, value, valuetype	-

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