HTML5 + CSS

HTML5 is an umbrella term that contains both HTML markup language and some adjoining technologies used to develop web pages and applications. It’s an evolving, living standard managed W3C and WHATWG. Along with languages like JavaScript and CSS it allows to define the contents of web pages, how they look and interact with the user.

It's commonly known

All developers would have come across it at some point in their work.

It allows you to create apps quickly and easily

It’s a cross-platform tool that allows you to create applications quickly and (relatively) easily.

It's extremely flexible

At this moment there are virtually no user-facing applications that can’t be implemented using HTML5 (along with CSS and JS) as the presentation layer.