Overview
If a rendered image has a significantly different aspect ratio from the aspect ratio in its source file (the "natural" aspect ratio), then the rendered image may look distorted, possibly creating an unpleasant user experience.
Recommendations
- Avoiding setting the width or height of an element as a percentage of a variably-sized container.
- Avoid setting explicit width or height values that differ from the source image's dimensions.
- Consider using css-aspect-ratio or Aspect Ratio Boxes to help preserve aspect ratios.
- When possible, it's a good practice to specify image width and height in HTML, so that the browser can allocate space for the image, which prevents it from jumping around as the page loads. It's more optimal to specify width and height in HTML rather than CSS, because the browser allocates space before parsing the CSS. In practice this approach can be difficult if you're working with responsive images, because there's no way to specify width and height until you know the viewport dimensions.
Inspecting rendered images with incorrect aspect ratios
Chrome DevTools can show you the CSS declarations that affect an image's aspect ratio. See View only the CSS that's actually applied to an element.
More information
Lighthouse flags any image that has a rendered aspect ratio which is 5 percent or more different than its natural ratio.