HTML Guide
An <iframe> element allows to embed an HTML document inside another HTML document, and its src attribute is indicated the source URL of the embedded web page. The query part of that URL contains one or more space characters, which are not allowed, for example:
<iframe src="https://maps.google.it/maps?q=2700 6th Avenue"></iframe>
You should properly escape all space characters as %20 like this:
<iframe src="https://maps.google.it/maps?q=2700%206th%20Avenue"></iframe>Learn more:
Related W3C validator issues
An <iframe> element allows to embed an HTML document inside another HTML document, and its src attribute is indicated the source URL of the embedded web page. The src attribute is a required attribute, so it cannot be blank.
Example:
<iframe src="https://example.com/map.html"></iframe>The issue arises from the space character in the src attribute value of the script element. In URLs, spaces are not allowed and should be properly encoded to avoid validation errors.
Fix
Replace spaces with %20, which is the URL-encoded representation of a space.
Example
Before:
<script src="https://example.com/media assets/app.js"></script>After:
<script src="https://example.com/media%20assets/app.js"></script>Explanation
In this example, the space between “media” and “assets” in the URL is replaced with %20. This change ensures that the URL conforms to standards and is correctly processed by browsers and servers. Spaces and other special characters in URLs must be encoded to ensure proper formatting and accessibility.
The <script> tag allows authors to include dynamic scripts and data blocks in their documents. When the src is present, this tag accepts a type attribute which must be either:
- an empty string
- text/javascript (that’s the default, so it can be omitted)
- module
Examples:
<!-- This is valid, without a type it defaults to JavaScript -->
<script src="app.js"></script>
<!-- This is valid, but will warn that it can be omitted -->
<script type="text/javascript" src="app.js"></script>
<!-- An empty attribute is valid, but will warn that it can be omitted -->
<script type="" src="app.js"></script>
<!-- The module keyword is also valid as a type -->
<script type="module" src="app.js"></script>
<!-- Any other type is invalid -->
<script type="wrong" src="app.js"></script>
<script type="text/html" src="app.js"></script>
<script type="image/jpeg" src="app.js"></script>The seamless attribute was proposed to be included in the HTML5 spec, but it wasn’t finally accepted, so it’s not a valid attribute for <iframe>.
There is an iframe tag inside a noscript tag that is itself inside the head section of the HTML document. This is not allowed because an iframe cannot be nested inside the head section.
To fix this issue, you may move the noscript section that contains the iframe tag outside of the head section, and ensure that it is placed within the body section of the HTML document.
For example, this is invalid HTML because the head section cannot contain iframe elements:
<!DOCTYPE html>
<html lang="en">
<head>
<title>My webpage</title>
<noscript>
<p>Please enable JavaScript to view this website</p>
<iframe src="https://example.com/"></iframe>
</noscript>
<!-- Other meta tags and styles go here -->
</head>
<body>
<!-- Rest of your webpage content goes here -->
</body>
</html>Moving the noscript inside the body section fixes the issue, as that’s where iframe elements belong:
<!DOCTYPE html>
<html lang="en">
<head>
<title>My webpage</title>
<!-- Other meta tags and styles go here -->
</head>
<body>
<noscript>
<p>Please enable JavaScript to view this website</p>
<iframe src="https://example.com/"></iframe>
</noscript>
<!-- Rest of your webpage content goes here -->
</body>
</html>The src attribute on an <img> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.
The src attribute for <img> tags is required, to define the source of the image, like in this example:
<img src="photo.jpg" alt="wombat" />Ensure the src attribute on the script element is non-empty and points to a valid resource.
The src attribute in a script element specifies the URL of an external script file. An empty src attribute is invalid because it tells the browser to fetch a resource from a URL that is not provided, leading to loading errors. Instead, ensure that the src attribute contains a valid file path or URL to an existing script file. If the script content is meant to be inline, you should omit the src attribute altogether and include the script content directly within the script element.
Example of a Valid External Script
Here is a valid example of a script element with a non-empty src attribute:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Valid Script Example</title>
</head>
<body>
<script src="path/to/script.js"></script>
</body>
</html>Example of a Valid Inline Script
If the script is to be written inline, exclude the src attribute and write the JavaScript code directly within the script tags:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Inline Script Example</title>
</head>
<body>
<script>
console.log('This is an inline script.');
</script>
</body>
</html>Troubleshooting
Double-check the script’s file path:
- Ensure the file path you provide in the src is correct relative to the HTML file.
- Make sure the script file exists in the location specified.
- If using a network URL, verify that the URL is correct and accessible.
The attributes width and height of <iframe> elements expect a non-negative integer, so an empty string is not allowed. Either define the correct dimension, or remove this attribute.
The allowfullscreen attribute is used to allow an iframe to activate fullscreen mode. As a boolean attribute, it should only be declared without any value.
Here is an example of correct usage:
<iframe src="https://example.com" allowfullscreen></iframe>However, this is now a legacy attribute, and has been redefined as allow="fullscreen", as part of the more general Permissions Policy:
<iframe src="https://example.com" allow="fullscreen"></iframe>