Few things are as annoying on the web as having the page layout unexpectedlychange or shift while you're trying to view or interact with it. This layout shifting is especially frustrating on mobile devices, where a big enough change can push all familiar content off screen and cause a visitor to completely lose her bearings.
These are both things that you do to assets on your website. They are both things that reduce the size of the file, making it more efficient in crossing the network between servers and browsers. But these two things are distinctly different. If you didn't already know that, it's worth understanding.
When HTML was announced by Tim Berners-Lee in 1991 there was no method of styling pages. How given HTML tags were rendered was determined by the browser, often with significant input from the user’s preferences. It seemed, however, like a good idea to create a standard way for pages to ‘suggest’ how they might prefer to be rendered stylistically.