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:
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A CMS or templating engine outputs width="" or height="" because no dimension value was configured.
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JavaScript dynamically sets img.setAttribute("width", "") instead of removing the attribute.
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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.