Description:
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The HTML that is generated by DomDocument::saveHTML() (and probably saveHTMLFile() too) does not validate as XHTML (as in '1.0 Transitional'). In stead it follows common practice for HTML4.
Since some time, it's Good Practice (TM) to use simple rules like:
- always give a value to an attribute
- close single-tag elements with a '/>' instead of '>'
 etc.

All common browsers understand this syntax, even when a HTML 4.0 DTD is specified.

With the current implementation, it's impossible to create valid XHTML (snippets) using the php5 DOM extension.

Reproduce code:
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$doc = new DomDocument();
$input = $doc->createElement('input');
$input->setAttribute('type', 'checkbox');
$input->setAttribute('checked', 'checked');
$doc->appendChild($input);
echo $doc->saveHTML();

Expected result:
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This should generate a string like:
<input type="checkbox" checked="checked" />
which is valid XHTML.

Actual result:
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The generated string looks like this: 
<input type="checkbox" checked>
This code gives 2 parse errors:
- no value for attribute 'checked'
- end tag for 'input' omitted (i,e, missing the forward slash)