Use fluid grids, flexible images, and CSS media queries to ensure your site looks great on all devices. Mobile-first design is highly recommended for modern development.

Optimize images, minify CSS/JS, implement lazy loading, use caching, and consider a CDN. Regularly audit with tools like Google Lighthouse for actionable insights.

Accessibility ensures everyone, including people with disabilities, can use your website or app. This expands your audience and may be required by law in many regions.

PWAs are web apps that use modern web capabilities to deliver an app-like experience. They work offline, can be installed, and offer improved speed and engagement.

Use HTTPS, validate user input, implement authentication & authorization, regularly update dependencies, and follow OWASP security best practices.