Are empty HTML5 data attributes valid?
Yes, it is valid syntax to omit value for a custom data attribute.
"Attributes can be specified in four different ways:
Empty attribute syntax Just the attribute name. The value is implicitly the empty string. [...]" https://developers.whatwg.org/syntax.html#attributes-0
On one hand, it passes the validator 16.5.7 https://validator.w3.org/nu/#textarea :
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8"/>
<title>a</title>
</head>
<body data-asdf>
</body>
</html>
On the other, HTML5 does not say in the specification of data- attributes that they are boolean: https://www.w3.org/TR/html5/dom.html#custom-data-attribute while it says that very clearly for other boolean attributes like checked https://www.w3.org/TR/html5/forms.html#attr-input-checked
Yes, perfectly valid. In your case, data-modal-target would represent a boolean attribute:
2.4.2 Boolean attributes
The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.
Valid, but they aren't boolean.
Custom data attributes specification doesn't mention any changes to empty attributes handling, so the general rules about empty attributes apply here:
Certain attributes may be specified by providing just the attribute name, with no value.
In the following example, the
disabledattribute is given with the empty attribute syntax:<input disabled>Note that empty attribute syntax is exactly equivalent to specifying the empty string as the value for the attribute, as in the following example.
<input disabled="">
So you are allowed to use empty custom data attributes, but special handling is needed to use them as boolean.
When you are accessing an empty attribute, its value is "". Since it's a falsy value, you can't just use if (element.dataset.myattr) to check whether an attribute is present.
You should use element.hasAttribute('myattr') or if (element.dataset.myattr !== undefined) instead.
Lloyd's answer is wrong. He mentions link to boolean attributes microsyntax, but data-* attributes are not specified as boolean in the spec.