HTML Guide
The W3C HTML Validator issue you encountered indicates that the value of the height attribute for an <img> element is set to “100%”, which is not allowed in HTML. The height attribute must be a specific digit that represents the height in pixels (e.g., “150”), rather than a percentage or any other unit.
How to Fix the Issue
To resolve this issue, you need to specify a numeric value for the height attribute instead. If you want the image to occupy a certain percentage of the container, use CSS instead of the height attribute.
Example of Incorrect HTML
Here is an example that causes the validation issue:
<img src="image.jpg" height="100%" alt="Example Image">Corrected HTML Using Pixel Values
To correct the issue, replace the percentage with a pixel value:
<img src="image.jpg" height="200" alt="Example Image">Alternative: Using CSS for Responsive Design
If you want the image to scale responsively and occupy 100% of the width of its container, consider using CSS:
<style>
.responsive-img {
width: 100%;
height: auto; /* Maintains aspect ratio */
}
</style>
<img src="image.jpg" class="responsive-img" alt="Example Image">Summary
-
Do not use percentage values for the
heightattribute of<img>tags. - Use pixel values for fixed dimensions.
- For responsive design, use CSS to set the dimensions instead.
Learn more:
Related W3C validator issues
The attributes width and height of <img> elements expect a non-negative integer, so an empty string is not allowed. Either define the correct dimension, or remove this attribute.
The attributes width and height on an iframe expect a valid positive integer without any decimals.
Here’s an example of incorrect code where decimals are being used for dimension attributes:
<img src="photo.jpg" alt="Dog" height="602.88" width="800.2">Corrected code without decimals:
<img src="photo.jpg" alt="Dog" height="603" width="800">In the corrected code, the width and height values has been changed to a whole number, which conforms to the standard integer value expected by the W3C validator.
The width and height attributes on <img> and <iframe> elements expect a digit to indicate the number of pixels. Ensure that this attribute contains only digits.
For example:
<!-- This is invalid because width is not a digit -->
<img width="225px" height="100px" alt="cat" src="cat.jpg" />
<!-- This is valid -->
<img width="225" height="100" alt="cat" src="cat.jpg" /><img> tags, used to include images on a document, require an alt attribute to describe the contents of the image. This is essential for users that cannot see the image (like screen reader users), or as an alternate text when the image cannot be displayed. Example:
<img src="photo.jpg" alt="Person holding a cat" />When an img element has an empty alt attribute, its role is implicitly decorative, so it must not specify a role attribute.
The <table> element does not accept a height attribute. Use CSS instead.
A start tag for <img> has been found inside a <noscript> section within the <head>, where it’s not allowed. Consider moving it to the <body> section.
The HTML <noscript> element defines a section of HTML to be inserted if a script type on the page is unsupported or if scripting is currently turned off in the browser.
When JavaScript is disabled, the content inside <noscript> will be used instead, so this content must fit within its parent section. As an <img> tag is not allowed inside <head>, this will raise an issue. Instead, consider moving the <noscript> part to the <body> section.
This issue is often related to 3rd party tracking pixels like the Facebook or LinkedIn conversion tracking pixels. For example, the Facebook pixel instructions tell you to insert it like this:
<html>
<head>
<script>
...some script...
</script>
<noscript>
<img src="..." />
</noscript>
</head>
<body>
...
</body>
</html>Instead, consider moving the <noscript> part inside the <body>, where the <img> makes sense to be inserted:
<html>
<head>
<script>
...some script...
</script>
</head>
<body>
...
<noscript>
<img src="..." />
</noscript>
</body>
</html>The width and height attributes on <img> elements expect a digit to specify the dimension in pixels. It should not contain units, letters or percent signs.
You can achieve this using CSS instead, for example:
<!-- Invalid syntax, the height attribute expects only digits -->
<img src="photo.jpg" alt="cat" height="auto" />
<!-- Valid syntax using CSS -->
<img src="photo.jpg" alt="cat" style="height: auto" />The sizes attribute specifies the size of the image when it is displayed on different devices.
The error message is saying that the value auto is not a valid value for the sizes attribute.
To fix this issue, you need to replace the value auto with a valid size. You can use a width descriptor or a media query to specify the size for different device widths.
Here’s an example of using a width descriptor:
<img src="example.jpg" sizes="(max-width: 600px) 100vw, 50vw" />This example sets the size of the image to 100% of the viewport width when the device width is less than or equal to 600px, and 50% of the viewport width for larger device widths.
Alternatively, you can remove the sizes attribute altogether and let the browser decide the best size for the image based on the viewport size.
<img src="example.jpg" />If you do this, the browser will use the default sizes value of 100vw and will scale the image accordingly.
The src attribute on an <img> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.