About This HTML Issue
In HTML, boolean attributes work differently from regular attributes. They don’t take true or false as values. Instead, the mere presence of the attribute represents the “true” state, and its complete absence represents “false.” According to the WHATWG HTML specification, a boolean attribute has exactly three valid representations: the attribute name alone (checked), an empty string value (checked=""), or the attribute name as its own value (checked="checked"). Any other value — including seemingly intuitive ones like "true", "false", "yes", or "no" — is invalid.
This is particularly important to understand because checked="false" does not uncheck the input. Since the attribute is still present, the browser interprets it as checked. This can lead to confusing behavior where a developer writes checked="false" expecting the checkbox to be unchecked, but it renders as checked. To leave an input unchecked, you must omit the checked attribute entirely.
The checked attribute applies to <input> elements of type checkbox and radio. It sets the initial checked state when the page loads. Note that JavaScript uses a different convention — the checked DOM property accepts true or false as JavaScript boolean values — but this does not apply to HTML markup.
Fixing this issue ensures standards compliance, avoids unexpected behavior, and improves code clarity. While browsers are lenient and will typically treat any value as “checked,” relying on this behavior is non-standard and can cause confusion for developers reading the code.
Examples
Invalid: using a string value
<input type="checkbox" checked="true">
<input type="radio" name="color" value="red" checked="yes">
These produce validation errors because "true" and "yes" are not valid boolean attribute values.
Invalid: attempting to set false
<!-- This does NOT uncheck the input — it's still checked! -->
<input type="checkbox" checked="false">
Despite the value "false", the checkbox will still render as checked because the attribute is present.
Valid: attribute name only (recommended)
<input type="checkbox" checked>
<input type="radio" name="color" value="red" checked>This is the most common and cleanest form.
Valid: empty string value
<input type="checkbox" checked="">Valid: attribute name as its own value
<input type="checkbox" checked="checked">This form is sometimes seen in XHTML-compatible markup.
Valid: omitting the attribute to leave unchecked
<input type="checkbox">To represent an unchecked state, simply leave the attribute off.
Full example in context
<!DOCTYPE html>
<html lang="en">
<head>
<title>Newsletter Preferences</title>
</head>
<body>
<form>
<fieldset>
<legend>Email preferences</legend>
<label>
<input type="checkbox" name="newsletter" checked>
Subscribe to newsletter
</label>
<label>
<input type="checkbox" name="promotions">
Receive promotional emails
</label>
</fieldset>
<fieldset>
<legend>Frequency</legend>
<label>
<input type="radio" name="frequency" value="daily" checked>
Daily
</label>
<label>
<input type="radio" name="frequency" value="weekly">
Weekly
</label>
</fieldset>
</form>
</body>
</html>
In this example, the newsletter checkbox and the “Daily” radio button are pre-selected using the valid checked attribute without any value. The promotions checkbox and “Weekly” radio button are unchecked because the attribute is absent.
Find issues like this automatically
Rocket Validator scans thousands of pages in seconds, detecting HTML issues across your entire site.