Browsers can resize images based on the attributes you specify.
However I imagine most resizing is done server side, preferably prior to the request. With the speed of current CPUs and for the sake of simplicity and possibly bandwidth saving, is it ever acceptable to leave image resizing to the browser?
One use-case is when you have a thumbnail and a full-size image, or various sizes to display. In that case it is fine to use one image and resize. Note that modern browsers now use bicubic resampling, so you don’t run into jaggy, cruddy interpolations with lots of artifacts when you resize, which used to be a very compelling reason NOT to let the browser resize the image.