HTML Guides for multiple
Learn how to identify and fix common HTML validation errors flagged by the W3C Validator — so your pages are standards-compliant and render correctly across every browser. Also check our Accessibility Guides.
In HTML, boolean attributes work differently than you might expect from programming languages. A boolean attribute’s presence on an element represents the true value, and its absence represents false. You don’t set them to "true" or "false" like you would in JavaScript or other languages. According to the WHATWG HTML specification, a boolean attribute has exactly three valid forms:
- The attribute name alone (e.g., multiple)
- The attribute with an empty value (e.g., multiple="")
- The attribute with a value matching its own name, case-insensitively (e.g., multiple="multiple")
Setting multiple="true" is invalid because "true" is not one of the permitted values. While browsers are forgiving and will typically still treat the attribute as present (effectively enabling it), this produces a W3C validation error and does not conform to the HTML standard. Relying on browser leniency leads to inconsistent behavior, makes your code harder to maintain, and can cause problems with HTML processing tools or strict parsers.
This same rule applies to all boolean attributes in HTML, including disabled, readonly, checked, required, hidden, autoplay, and many others.
It’s also important to note that multiple="false" does not disable the attribute. Because the attribute is still present on the element, the browser treats it as enabled. To disable a boolean attribute, you must remove it from the element entirely.
Examples
❌ Invalid: using "true" as the value
<label for="colors">Select your favorite colors:</label>
<select id="colors" name="colors" multiple="true">
<option value="red">Red</option>
<option value="green">Green</option>
<option value="blue">Blue</option>
</select>This triggers the validation error: Bad value “true” for attribute “multiple” on element “select”.
✅ Fixed: attribute name only (preferred)
<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>
</select>✅ Fixed: empty string value
<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>
</select>✅ Fixed: value matching the attribute name
<label for="colors">Select your favorite colors:</label>
<select id="colors" name="colors" multiple="multiple">
<option value="red">Red</option>
<option value="green">Green</option>
<option value="blue">Blue</option>
</select>❌ Common mistake: using "false" to disable
<!-- This does NOT disable multiple selection — the attribute is still present -->
<select name="colors" multiple="false">
<option value="red">Red</option>
<option value="green">Green</option>
</select>✅ Correct way to disable: remove the attribute entirely
<label for="color">Select a color:</label>
<select id="color" name="color">
<option value="red">Red</option>
<option value="green">Green</option>
</select>Invalid value used for the multiple attribute on an input element.
The multiple attribute is a boolean attribute for certain input types (e.g., email, file). Boolean attributes must appear without a value (or with the same name as value in legacy cases), and they only work on specific types. Valid usage: <input type="email" multiple> or <input type="file" multiple>. Invalid usage includes multiple="1", multiple="true", or using multiple on unsupported types like text or number. The email type allows comma-separated addresses when multiple is present; the file type allows selecting more than one file.
HTML Examples
Example that reproduces the error
<!DOCTYPE html>
<html lang="en">
<head>
<title>Invalid multiple</title>
</head>
<body>
<!-- Invalid: value on boolean attribute, and wrong type -->
<input type="text" name="tags" multiple="1">
</body>
</html>Corrected example
<!DOCTYPE html>
<html lang="en">
<head>
<title>Valid multiple</title>
</head>
<body>
<!-- Valid: boolean attribute without a value on supported types -->
<input type="email" name="recipients" multiple placeholder="name@example.com, other@example.com">
<input type="file" name="attachments" multiple>
</body>
</html>A <select> element can only have multiple <option selected> if it includes the multiple attribute.
A <select> element represents a control that provides a menu of options. By default, only one option can be selected at a time unless the multiple attribute is present, allowing users to select more than one option. If multiple <option> elements use the selected attribute without multiple, this violates the HTML standard and triggers a validation error.
Example of incorrect usage:
<select name="color">
<option value="red" selected>Red</option>
<option value="green" selected>Green</option>
<option value="blue">Blue</option>
</select>Correct usage with only one selected option:
<select name="color">
<option value="red" selected>Red</option>
<option value="green">Green</option>
<option value="blue">Blue</option>
</select>Correct usage for multiple selected options with multiple attribute:
<select name="color" multiple>
<option value="red" selected>Red</option>
<option value="green" selected>Green</option>
<option value="blue">Blue</option>
</select>Remove duplicate selected attributes unless multiple is set, or add the multiple attribute if multiple selection is intended.
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