Well-Formedness - Refactoring HTML: Improving the Design of Existing Web Applications - page 74

Fill In Omitted Attribute Value
Add values to all valueless attributes.
<input type="radio" name="p" value="debit" checked></input>
<input name="generator" value="system78" readonly></input>
<input name="date" value="2007-12-17" disabled></input>
<a href="http://example.com/imagemap/library">
<img src="duane.png" ismap></a>
<input type="radio" name="p" value="debit" checked="checked" />
<input name="generator" value="system78" readonly='readonly' /
<input name="date" value="2007-12-17" disabled="disabled"></input>
<a href="http://example.com/imagemap/library">
<img src="duane.png" ismap="ismap"></a>
Motivation
XHTML does not support the attribute name-only syntax.
Potential Trade-offs
Minimal. Browsers are perfectly happy to read the values you supply.
Mechanics
Omitted attribute values are fairly rare in practice. The only place they're at all common is in forms and image
maps. It may well be possible to manually fix all occurrences on a site without a great deal of effort.
Alternatively, you can just use Tidy or TagSoup.
To fix one, just set the attribute value to the name. For example, change this:
<input type="radio" name="p" checked>
into this:
<input type="radio" name="p" checked='checked'>
Only a few elements and attributes support valueless attributes in the first place:
input : checked , disabled , readonly , ismap
optgroup : disabled