Bad value “X%” for attribute “height” on element “video”: Expected a digit but saw “%” instead.

The height attribute on <video> is defined in the HTML specification as a “valid non-negative integer.” This means it must be a string of digits (e.g., "480") with no units, no decimal points, and no percentage signs. When you write something like height="50%", the W3C validator rejects it because % is not a digit character and percentage values are not part of the attribute’s allowed syntax.

This same rule applies to the width attribute on <video>, as well as the width and height attributes on <img>, <canvas>, and other elements that accept pixel dimensions. The integer you provide represents a size in CSS pixels, so height="480" means 480 CSS pixels — no unit suffix is needed or allowed.

Why this matters

• Standards compliance: The HTML living standard explicitly defines these attributes as non-negative integers. Using percentages violates the spec and produces a validation error.
• Browser behavior: While some browsers may attempt to interpret a percentage value, the behavior is undefined and inconsistent. You cannot rely on it working correctly across all browsers and versions.
• Layout stability: Providing valid width and height integer values helps browsers reserve the correct aspect ratio for the video before it loads, reducing cumulative layout shift (CLS). Invalid values undermine this benefit.

How to fix it

1. Replace the percentage value with a plain integer representing the desired pixel height.
2. If you need percentage-based or responsive sizing, remove the percentage from the HTML attribute and use CSS instead. CSS width and height properties fully support percentage values, viewport units, and other flexible sizing methods.
3. Always keep integer width and height attributes on the element when possible, even if you also use CSS for responsive sizing. This provides an intrinsic aspect ratio hint to the browser.

Examples

❌ Invalid: percentage value in the height attribute

<video controls width="100%" height="50%">
  <source src="/media/video.webm" type="video/webm">
</video>

The validator reports: Bad value “50%” for attribute “height” on element “video”: Expected a digit but saw “%” instead. The same error applies to the width="100%" attribute.

✅ Fixed: integer values for intrinsic dimensions

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

Both width and height use plain integers representing CSS pixels. This is valid and gives the browser an aspect ratio hint (4:3 in this case).

✅ Fixed: responsive sizing with CSS

If you need the video to scale as a percentage of its container, use CSS for the sizing while keeping valid integer attributes for the aspect ratio:

<video controls width="640" height="360" style="width: 100%; height: auto;">
  <source src="/media/video.webm" type="video/webm">
</video>

Here, width="640" and height="360" tell the browser the video’s intrinsic aspect ratio (16:9), while the inline style (or an external stylesheet) makes the video fill 100% of its container width and scale its height proportionally. This approach is both valid HTML and fully responsive.

✅ Fixed: using a CSS class for responsive video

<style>
  .responsive-video {
    width: 100%;
    max-width: 800px;
    height: auto;
  }
</style>

<video controls width="800" height="450" class="responsive-video">
  <source src="/media/video.webm" type="video/webm">
</video>

This keeps the HTML valid, provides an aspect ratio hint, and achieves flexible, percentage-based sizing entirely through CSS.

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