background-transition with background-size: cover

Question

This may already be answered somewhere but I haven\'t found yet after a bit of searching.

I have a series of divs with background-images. The size is set to backgrou

Answer 1

You can't use keywords (such as cover) when using CSS animations for background-size.

More info here: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_animated_properties

Relevant text:

background-size - yes, as a repeatable list of a simple list of a length, percentage or calc(); when both values are lengths, they are interpolated as lengths; when both values are percentages, they are interpolated as percentages; otherwise, both values are converted into a calc() function that is the sum of a length and a percentage (each possibly zero), and these calc() functions have each half interpolated as real numbers. . This means keyword values are not animatable.

One approach to get this effect is to place element with the background image in a wrapping element with overflow hidden and apply a scale transform.

.wrapper { 
  width:300px;
  height:400px;
  overflow:hidden;
}
.image {
  background:url("http://placekitten.com/g/500/500");
  background-size:cover;
  width:100%;
  height:100%;
  transition: transform 2s;
}

.image:hover { transform:scale(1.1) }