HTML Guide for multiple
The multiple attribute is used to indicate that multiple options can be selected in a <select> element. As a boolean attribute, it should only be declared without any value.
Instead of:
<select multiple="true">You should use:
<select multiple>Here is an example of the correct usage of the multiple attribute:
<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>
<option value="yellow">Yellow</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.