Bad value “” for attribute “(width|height)” on element “iframe”: The empty string is not a valid non-negative integer.

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.

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 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 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>

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 <iframe> element, used to embed another document inside the current document, accepts both attributes width and height which must be valid non-negative integers. Percentages are not allowed for these attributes.

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

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>.