About This HTML Issue
The multiple attribute tells the browser that the user can supply more than one value for a given input. For <input type="file">, it allows selecting multiple files at once. For <input type="email">, it allows entering a comma-separated list of email addresses. These are the only two input types that support the multiple attribute according to the HTML specification.
This validation error can appear for two distinct reasons, and sometimes both at once:
-
An invalid value is assigned to the boolean attribute. Boolean attributes in HTML follow strict rules. Valid syntaxes are: the attribute name alone (
multiple), an empty string value (multiple=""), or the attribute’s own name as the value (multiple="multiple"). Any other value — includingmultiple="true",multiple="1", ormultiple="yes"— is invalid. -
The attribute is used on an unsupported input type. Placing
multipleon input types liketext,number,password, orurlis not valid because those types don’t define behavior for this attribute. Browsers will simply ignore it, but it still constitutes invalid markup.
Why this matters
-
Standards compliance: Invalid boolean attribute values violate the WHATWG HTML specification. While most browsers are forgiving and may still interpret
multiple="true"as the attribute being present, relying on this behavior is fragile and non-standard. - Accessibility: Assistive technologies rely on valid, well-structured markup. An invalid attribute value could lead to unpredictable behavior in screen readers or other tools.
-
Maintainability: Using
multipleon an unsupported input type suggests a misunderstanding of the element’s capabilities, which can confuse other developers and lead to bugs.
How to fix it
-
Remove the value entirely: Change
multiple="1"ormultiple="true"to justmultiple. -
Use a valid boolean syntax if a value is required: Some templating systems or XML-based contexts (like XHTML) require explicit attribute values. In those cases, use
multiple=""ormultiple="multiple". -
Ensure the input type supports
multiple: Onlytype="email"andtype="file"accept this attribute. If you need multi-value input for other types, consider alternative approaches like multiple separate inputs, a<select multiple>element, or a JavaScript-based solution.
Examples
Invalid: wrong value on a boolean attribute
<!-- Bad: "1" is not a valid boolean attribute value -->
<input type="file" name="attachments" multiple="1">
<!-- Bad: "true" is not a valid boolean attribute value -->
<input type="email" name="recipients" multiple="true">
Invalid: multiple on an unsupported input type
<!-- Bad: type="text" does not support the multiple attribute -->
<input type="text" name="tags" multiple>
<!-- Bad: type="number" does not support the multiple attribute -->
<input type="number" name="quantities" multiple>
Valid: correct usage of multiple
<!-- Correct: boolean attribute with no value -->
<input type="file" name="attachments" multiple>
<!-- Correct: empty string value (valid boolean syntax) -->
<input type="email" name="recipients" multiple="">
<!-- Correct: attribute name as value (valid for XHTML compatibility) -->
<input type="file" name="documents" multiple="multiple">Full corrected document
<!DOCTYPE html>
<html lang="en">
<head>
<title>Upload Form</title>
</head>
<body>
<form action="/submit" method="post" enctype="multipart/form-data">
<label for="recipients">Recipients:</label>
<input type="email" id="recipients" name="recipients" multiple placeholder="a@example.com, b@example.com">
<label for="files">Attachments:</label>
<input type="file" id="files" name="files" multiple>
<button type="submit">Send</button>
</form>
</body>
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