HTML Guide for height

The <table> element does not accept a height attribute. Use CSS instead.

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 height attribute of <img> tags.
• Use pixel values for fixed dimensions.
• For responsive design, use CSS to set the dimensions instead.

The height attribute on the <video> element must be a non-negative integer representing the height in CSS pixels. The value “auto” is not a valid value for this attribute. To resolve the issue, set the height attribute to a specific numeric value or adjust the height using CSS instead.

Example with a numeric height value:

<video width="640" height="360" controls>
  <source src="movie.mp4" type="video/mp4">
  Your browser does not support the video tag.
</video>

Alternatively, control the height using CSS:

<video width="640" controls style="height: auto;">
  <source src="movie.mp4" type="video/mp4">
  Your browser does not support the video tag.
</video>

In the CSS approach, “auto” can be used, but it should not be part of the HTML attributes.

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 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 HTML specification requires that the width and height attributes on <img> elements, when present, contain a string representing a non-negative integer — that is, a sequence of one or more ASCII digits like "0", "150", or "1920". An empty string ("") does not satisfy this requirement, so the W3C validator flags it as an error.

This issue commonly arises when:

• A CMS or templating engine outputs width="" or height="" because no dimension value was configured.
• JavaScript dynamically sets img.setAttribute("width", "") instead of removing the attribute.
• A developer adds the attributes as placeholders intending to fill them in later but forgets to do so.

Why it matters

Providing valid width and height attributes is one of the most effective ways to prevent Cumulative Layout Shift (CLS). Browsers use these values to calculate the image’s aspect ratio and reserve the correct amount of space before the image loads. When the values are empty strings, the browser cannot determine the aspect ratio, so no space is reserved — leading to layout shifts as images load in, which hurts both user experience and Core Web Vitals scores.

Beyond performance, invalid attribute values can cause unpredictable rendering behavior across browsers. Some browsers may ignore the attribute, others may interpret the empty string as 0, collapsing the image to zero pixels in that dimension. Standards-compliant HTML also improves accessibility by ensuring assistive technologies can parse the document reliably.

Examples

❌ Invalid: empty string values

<img src="photo.jpg" alt="A sunset over the ocean" width="" height="">

Both width and height are set to empty strings, which is not valid.

✅ Fixed: provide actual dimensions

<img src="photo.jpg" alt="A sunset over the ocean" width="800" height="600">

Replace the empty strings with the image’s actual pixel dimensions. These values should reflect the image’s intrinsic (natural) size. CSS can still be used to scale the image visually — the browser will use the width and height ratio to reserve the correct space.

✅ Fixed: remove the attributes entirely

<img src="photo.jpg" alt="A sunset over the ocean">

If you don’t know the dimensions or prefer to handle sizing purely through CSS, remove the attributes altogether. An absent attribute is valid; an empty one is not.

❌ Invalid: only one attribute is empty

<img src="banner.jpg" alt="Promotional banner" width="1200" height="">

Even if only one attribute has an empty value, the validation error will be triggered for that attribute.

✅ Fixed: both attributes with valid values

<img src="banner.jpg" alt="Promotional banner" width="1200" height="400">

Fixing dynamic/template-generated markup

If a template language is outputting empty attributes, use a conditional to omit them when no value is available. For example, in a template:

<!-- Instead of always outputting the attributes: -->

<img src="photo.jpg" alt="Description" width="" height="">

<!-- Conditionally include them only when values exist: -->

<img src="photo.jpg" alt="Description" width="800" height="600">

If you’re setting dimensions via JavaScript, remove the attribute rather than setting it to an empty string:

// ❌ Don't do this
img.setAttribute("width", "");

// ✅ Do this instead
img.removeAttribute("width");

// ✅ Or set a valid value
img.setAttribute("width", "800");

A note on values

The width and height attributes only accept non-negative integers — whole numbers without units, decimals, or percentage signs. Values like "100px", "50%", or "3.5" are also invalid. Use plain integers like "100" or "600". If you need responsive sizing with percentages or other CSS units, apply those through CSS styles instead.

Remove the unit; height on embed expects a non-negative integer (pixels) or a valid CSS length only when set via CSS, not the HTML attribute.

Detailed explanation

The embed element supports the presentational attributes width and height as unsigned integers representing CSS pixels. In HTML, the height attribute must be a number without a unit, for example 650. Supplying 650px violates the attribute’s value syntax and triggers the validator error.

If you want to use units or other CSS lengths (e.g., px, em, %, vh), set them with CSS via the style attribute or a stylesheet using the height property, not the HTML attribute.

HTML examples

Example reproducing the issue

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Embed height invalid</title>
</head>
<body>
  <embed src="file.pdf" type="application/pdf" width="800" height="650px">
</body>
</html>

Corrected example (HTML attribute as integer)

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Embed height fixed</title>
</head>
<body>
  <embed src="file.pdf" type="application/pdf" width="800" height="650">
</body>
</html>

Alternative corrected example (use CSS units)

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Embed height via CSS</title>
  <style>
    .viewer { width: 800px; height: 650px; }
  </style>
</head>
<body>
  <embed class="viewer" src="file.pdf" type="application/pdf">
</body>
</html>

The value used in the height attribute on element iframe is not a valid integer. Remove any leading or trailing spaces from the attribute value.

Here’s an example:

<iframe width="560" height="315" src="your-video-link" frameborder="0" allowfullscreen></iframe>

<video> elements accept a height attribute to specify the width in CSS pixels. This value can only be an integer, it should not contain units or %. If you need to specify a percentage width, you can do that with CSS.

Here’s an example of setting width and height on a video element.

<video controls width="640" height="480">
  <source src="/media/cc0-videos/flower.webm" type="video/webm">
</video>

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:

<iframe src="example.html" height="602.88" width="800.2"></iframe>

Corrected code without decimals:

<iframe src="example.html" height="603" width="800"></iframe>

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 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.

Attribute values for width and height on the object element must be a valid integer (pixels), not a percentage.

In HTML5, the width and height attributes for the <object> element must be specified in pixels as an integer value, such as height="500". Using percentage values like height="100%" is not allowed in the attribute according to the HTML living standard and will trigger a validation error. To set a percentage height, use the CSS width and height properties instead.

Correct usage (pixels via attribute):

<object data="example.pdf" type="application/pdf" width="600" height="400"></object>

Correct usage (percentage via CSS):

<object data="example.pdf" type="application/pdf" style="width: 100%; height: 100%;"></object>

Example of a full valid HTML document using CSS for 100% height:

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Object Height Example</title>
  <style>
    html, body {
      height: 100%;
      margin: 0;
    }
    object {
      height: 100%;
      width: 100%;
      display: block;
    }
  </style>
</head>
<body>
  <object data="example.pdf" type="application/pdf"></object>
</body>
</html>

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" />

The value for the height property in your CSS must be a valid CSS length or percentage, or the keyword auto.

height in CSS accepts values such as px, em, rem, %, or the keyword auto. Incompatible values, like an unrecognized string or a missing unit (e.g., just height: 100;), will trigger this validation error. Always specify a valid unit unless using 0, which doesn’t require a unit, or use a percentage if the parent element has a defined height.

Correct usage examples:

<style>
  .box {
    height: 200px;
  }
  .container {
    height: 70%;
  }
  .image {
    height: auto;
  }
  .zero {
    height: 0;
  }
</style>

Incorrect usage examples:

<style>
  .bad {
    height: 100; /* missing unit */
  }
  .bad2 {
    height: big; /* invalid keyword */
  }
</style>

Always include a valid unit like px, %, em, or use auto for the height property.

The height property in your CSS containing invalid or too many values. The height property should have only one valid length, percentage, or keyword value.

Valid Values for height Property:

1. Length values: px, em, rem, etc. (e.g., 100px, 10em)
2. Percentage values: (e.g., 50%)
3. Keyword values: auto, max-content, min-content, fit-content, inherit, initial, unset

Example of Incorrect Usage:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
    .example {
      height: 100px 50px; /* Incorrect: Too many values */
    }
  </style>
  <title>Height Property Example</title>
</head>
<body>
  <div class="example">Content</div>
</body>
</html>

Example of Correct Usage:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
    .example {
      height: 100px; /* Correct: One valid value */
    }
  </style>
  <title>Height Property Example</title>
</head>
<body>
  <div class="example">Content</div>
</body>
</html>