Bad value “true” for attribute “multiple” on element “select”.

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:

1. The attribute name alone (e.g., multiple)
2. The attribute with an empty value (e.g., multiple="")
3. 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>

Learn more:

MDN Reference

MDN - select

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