Avoid security risks when using the WordPress REST API plugin (soon to be in core).
Learn how to test, track, and increase the speed of your website.
Taking responsive design beyond CSS gives you control over your display in a content-specific way.
Slow pages have poor SEO ratings. There are plugins that can improve site performance, and your choice of hosting provider can have the greatest impact on the speed of your site.
Customizer is WordPress’s Theme Customization API and is becoming part of core soon.