HTML Guide for boolean attribute
All HTML elements may have the hidden boolean attribute set. When specified on an element, it indicates that the element is not yet, or is no longer, relevant, so browsers won’t render it.
Boolean attributes don’t accept values, its presence represents the true value and its absence represents the false value.
<!-- This is invalid because the hidden attribute should not have a value set -->
<div hidden="false"></div>
<!-- The correct way to hide a div is like this -->
<div hidden>This will be hidden</div>
<!-- And to show the element, we just don't hide it -->
<div>This won't be hidden</div>The allowfullscreen attribute is used to allow an iframe to activate fullscreen mode. As a boolean attribute, it should only be declared without any value.
Here is an example of correct usage:
<iframe src="https://example.com" allowfullscreen></iframe>However, this is now a legacy attribute, and has been redefined as allow="fullscreen", as part of the more general Permissions Policy:
<iframe src="https://example.com" allow="fullscreen"></iframe>Use the boolean attribute disabled without any value or with the attribute name as the value; "true" is invalid.
Detailed explanation
The HTML boolean attribute disabled indicates that the element is not editable, focusable, or submittable. For boolean attributes (per the WHATWG HTML standard and MDN), the presence of the attribute means “on/true,” and its absence means “off/false.” Valid syntaxes are:
- disabled
- disabled=""
- disabled="disabled"
Invalid values include arbitrary strings like "true" or "false". Using disabled="true" triggers the validator error because boolean attributes must not use non-empty values other than the attribute name itself.
Relevant elements that support disabled include input, button, select, textarea, optgroup, option, fieldset, and form (partial behavior). For dynamic enabling/disabling, set or remove the attribute via JavaScript rather than assigning string values.
HTML examples
Reproduce the validator error (invalid)
<!DOCTYPE html>
<html lang="en">
<head>
<title>Invalid Disabled Example</title>
</head>
<body>
<form>
<input type="text" disabled="true">
<button type="submit" disabled="false">Submit</button>
</form>
</body>
</html>Fix (valid boolean attribute usage)
<!DOCTYPE html>
<html lang="en">
<head>
<title>Valid Disabled Example</title>
</head>
<body>
<form>
<!-- Presence means disabled -->
<input type="text" disabled>
<!-- Also valid: disabled="disabled" or disabled="" -->
<button type="submit" disabled="disabled">Submit</button>
</form>
</body>
</html>The multiple attribute is used to indicate that multiple options can be selected in a <select> element. As a boolean attribute, it should only be declared without any value.
Instead of:
<select multiple="true">You should use:
<select multiple>Here is an example of the correct usage of the multiple attribute:
<label for="colors">Select your favorite colors:</label>
<select id="colors" name="colors" multiple>
<option value="red">Red</option>
<option value="green">Green</option>
<option value="blue">Blue</option>
<option value="yellow">Yellow</option>
</select>The selected attribute on option elements is boolean, so it should not have any value associated.
To fix this issue, simply remove the value assigned to the selected attribute.
Instead of this:
<select>
<option selected="true">Option 1</option>
<option>Option 2</option>
<option>Option 3</option>
</select>Use this:
<select>
<option selected>Option 1</option>
<option>Option 2</option>
<option>Option 3</option>
</select>In the example above, we’ve removed the value assigned to the selected attribute on the first option element. This will specify that “Option 1” is the default option to be selected in the dropdown list.
checked is a boolean attribute and should not have a value; it must be written as just checked.
The checked attribute is used with <input> elements of type checkbox or radio. As a boolean attribute, it is either present (interpreted as true) or omitted (false). Adding any value, like checked="true", is not valid and causes a validation error. The correct usage is simply checked.
Incorrect example:
<input type="checkbox" checked="true">Correct example:
<input type="checkbox" checked>Example in context:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Checkbox Example</title>
</head>
<body>
<label>
<input type="checkbox" checked>
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</label>
</body>
</html>