Serving sharp images to high density screens - JakeArchibald.com
If you want your images to be as sharp as possible, you need to target images at the user's device pixels, rather than their CSS pixels.
Using rel="preconnect" to establish network connections early and increase performance | CSS-Tricks
Adding rel=preconnect to a informs the browser that your page intends to establish a connection to another domain, and that you'd like the process to start as soon as possible. Resources will load more quickly because the setup process has already been completed by the time the browser requests them.
Don’t use IDs in CSS selectors?