Bad value “1” for attribute “multiple” on element “select”.

The multiple attribute on the <select> element is a boolean attribute and does not accept a value like "1".

On a <select>, multiple lets the user pick more than one option. Like every boolean attribute in HTML, its meaning comes from whether it is present, not from what you set it to. Writing multiple="1" makes the W3C validator complain because "1" is not one of the values the spec allows.

A boolean attribute can only appear in three forms: the name on its own (multiple), an empty string (multiple=""), or the name repeated as the value (multiple="multiple"). Values such as "1", "true", or "yes" look reasonable but none of them are valid.

Invalid example

<select name="toppings" multiple="1">
  <option value="cheese">Cheese</option>
  <option value="mushrooms">Mushrooms</option>
  <option value="olives">Olives</option>
</select>

Valid example

<select name="toppings" multiple>
  <option value="cheese">Cheese</option>
  <option value="mushrooms">Mushrooms</option>
  <option value="olives">Olives</option>
</select>

Dropping the value and keeping just multiple is the clearest way to write it.

Learn more:

MDN Reference

HTML select element - MDN

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WHATWG Spec

WHATWG HTML Standard - Forms

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