Web performance: prevent wasteful hidden image requests (display: none)
We often hide images using CSS with "display: none", but this doesn't actually prevent the browser from downloading these images. Using the HTML Picture element, we can serve different versions of an image depending on media queries. We can even use a data-img to prevent images from being downloaded at all and optimize our website speed.
Updated Ajax + SEO Guidelines
When you build an Ajax based website and want to be SEO friendly, there are a couple of techniques you have to apply. Back in 2009, Google made a proposal on how to make your Ajax pages crawlable.