HTML Validation

HTTP resource not retrievable. The HTTP status from the remote server was: 503.

html
503

About This HTML Issue

The “HTTP resource not retrievable” error with an HTTP status code of 503 indicates that the W3C Validator could not access a web page referenced in your HTML. This error occurs when the remote server hosting the page is temporarily unavailable or overloaded.

Learn more:

→ MDN: 503 Service Unavailable

Last reviewed: April 22, 2025

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Related W3C Validator Issues

Attribute “isolang” not allowed on element “html” at this point.

Instead of using the isolang attribute to define the language of the document, you can use lang with an ISO 639-1 two character code.

For example, for Portuguese:

<html lang="pt">

Bad value X for attribute “multiple” on element “input”.

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>

HTTP resource not retrievable. The HTTP status from the remote server was: 429.

HTTP resource not retrievable. The HTTP status from the remote server was: 404.

Stray start tag “html”.

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