Most HTML elements are responsive by default, assuming we don't use pixel-based units with regard to their origin or size. Bitmap (or raster) images are a different story; they aren't resizable without losing information, so we have to deal with them separately.
There are three scenarios with respect to images:
Rendering an image with a specific size, such as an icon or a thumbnail
Rendering an image that needs to resize itself to the width of the viewport, as in a header image
Rendering a background image that needs to fill the viewport
In all these cases, we want the image to be as sharp as possible and require as little up sampling as possible, since this will result in blurry images. If we're rendering icons, we should also attempt to avoid down sampling as much as possible, since this can also result in poorly rendered icons.
When it comes to describing the differences between rendering on a display with a 1:1 pixel mapping (otherwise known as low density, loDPI, or nonretina...