Markup Languages: More Than HTML5 - Web Standards: Mastering HTML5, CSS3, and XML - page 64

Quoted Attribute Values
All attributes must include values in XHTML. All attribute values must be quoted (Listing 3-19). Unquoted attribute
values are not allowed in XHTML (Listing 3-20).

Listing 3-19. Defining XHTML Attributes Correctly
<input type="checkbox" name="checkbox" id="checkbox" value="True" checked="checked" />

Listing 3-20. Unquoted and Minimized Attributes (Incorrect in XHTML)

<input type=checkbox name=checkbox id=checkbox value=True checked />

No Attribute Minimization
Attribute-value pairs must be written in full (Listing 3-21). Attribute minimization is not supported by XHTML
(Listing 3-22). Attribute names such as compact and checked cannot be used in elements without specifying their
values (Table 3-3 ).

Listing 3-21. Attributes Should Be Written in Full
<option value="eng" selected="selected" >English</option>

Listing 3-22. Minimized Attribute (Incorrect in XHTML)

<option value="eng" selected>English</option>

Table 3-3. Attributes That Can Be Minimized in HTML, but Not in XHTML

Minimized Attribute (HTML) Full Form (HTML/XHTML)

compact compact="compact"

checked checked="checked"

declare declare="declare"

readonly readonly="readonly"

disabled disabled="disabled"

selected selected="selected"

defer defer="defer"

ismap ismap="ismap"

nohref nohref="nohref"

noshade noshade="noshade"

nowrap nowrap="nowrap"

multiple multiple="multiple"

noresize noresize="noresize"