Question

I’ve been reading some articles about HTML, XHTML, etc. In most of them (i.e. My preferred syntax style) say that boolean attributes should be written without any value, like this:

<input type="text" required>

They even say that it is wrong to use this attributes like this:

<input type="text" required="required">

Some of this articles link W3 which says:

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute’s canonical name, with no leading or trailing whitespace.

but in the examples shows like this:

Here is an example of a checkbox that is checked and disabled. The
checked and disabled attributes are the boolean attributes.

<label><input type=checkbox checked name=cheese disabled>Cheese</label>

This could be equivalently written as this:

<label><input type=checkbox checked=checked name=cheese disabled=disabled> Cheese</label>

You can also mix styles; the following is still equivalent:

<label><input type='checkbox' checked name=cheese disabled="">Cheese</label>

So, how should the boolean attributes be written? Based in your experience, which of the options are cross-browser and which are not?

Answer 1

Attribues without values are valid in HTML, but invalid in XHTML, because it’s not allowed in XML. Perhaps that’s where your confusion is coming from. So, which one is valid depends on the doctype of your document.