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What’s the Difference Between Width/Height in CSS and Width/Height HTML attributes?

Some HTML elements accept width and height as attributes. Some do not. For example:

<!-- valid, works, is a good idea -->
<img width="500" height="400" src="..." alt="...">
<iframe width="600" height="400" src="..."></iframe>
<svg width="20" height="20"></svg>

<!-- not valid, doesn't work, not a good idea -->
<div width="40" height="40"></div>
<span width="100" height="10"></span>

Those attributes are sometimes referred to as presentational attributes. The thing to know about them is that they are overridden by any other styling information whatsoever. That makes them ideal as a fallback.

So, if CSS loads and has a declaration like:

img {
  width: 400px;
}

...that is going to override the width="500" on the <img> tag above. Presentational attributes are the weakest kind of styling, so they are overridden by any CSS, even selectors with very low specificity.

What might be a smidge confusing is that presentational attributes seem like they would have high specificity. These inline styles, for instance, are very strong:

<img style="width: 500px; height: 400px;" src="..." alt="...">

Using an inline style (which works on any element, not a select few), we've moved from the weakest way to apply width and height to one of the strongest. Regular CSS will not override this, with a selector of any specificity strength. If we need to override them from CSS, we'll need !important rules.

img {
  width: 400px !important;
}

To reiterate, presentational attributes on elements that accept them (e.g. <img>, <iframe>, <canvas>, <svg>, <video>) are a good idea. They are fallback sizing and sizing information as the page is loading. They are particularly useful on <svg>, which may size themselves enormously in an awkward way if they have a viewBox and lack width and height attributes. Browsers even do special magic with images, where the width and height are used to reserve the correct aspect-ratio derived space in a situation with fluid images, which is great for a smooth page loading experience.

But presentational attributes are also weak and are usually overridden in the CSS.