aspect-ratio

The aspect-ratio  CSS property sets a preferred aspect ratio for the box, which will be used in the calculation of auto sizes and some other layout functions.

aspect-ratio: 1 / 1;

/* Global values */
aspect-ratio: inherit;
aspect-ratio: initial;
aspect-ratio: unset;

<auto>

Replaced elements with an intrinsic aspect ratio use that aspect ratio, otherwise the box has no preferred aspect ratio. Size calculations involving intrinsic aspect ratio always work with the content box dimensions.

<ratio>

The box’s preferred aspect ratio is the specified ratio of width / height. Size calculations involving preferred aspect ratio work with the dimensions of the box specified by box-sizing.

auto | <ratio>

aspect-ratio: 1 / 1;
aspect-ratio: 16 / 9;

Browsers have added an internal aspect-ratio property that applies to replaced elements and other related elements that accept width and height attributes. This appears in the browser's internal UA stylesheet. 

In Firefox, the internal stylesheet rule looks like this:

img, input[type="image"], video, embed, iframe, marquee, object, table {
  aspect-ratio: attr(width) / attr(height);
}

You can read more about this feature in Setting Height And Width On Images Is Important Again.

• Mapping the width and height attributes of media container elements to their aspect-ratio
• Designing an aspect ratio unit for CSS