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

1. 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)

2. All elements which can have end tags must have end tags

   e.g. <p>Paragraph</p><p>More</p> not <p>Paragraph <p>More

3. 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="" />

4. 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.

5. Attribute values must always be enclosed in quotes

   e.g. <td colspan="2"> not <td colspan=2>

6. All attributes must have values

   e.g. <input type="checkbox" name="choose" checked="checked" /> not <input type="checkbox" name="choose" checked />

7. Anchors used for jumping between links should have an id attribute not a name

   <a href="#foo">Go to foo</a> <a id="foo">Foo</a> Also, ids must match the pattern [A-Za-z][A-Za-z0-9:_.-]*

8. All ampersands should be encoded as &amp;

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

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.

Deprecated presentational tags - use CSS instead!

Deprecated structural tags

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.