HTML Guide for checked
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, not checked="true" or checked="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>