SVG

Scalable Vector Graphics has been a stable web technology for quite some time now … yet are relatively unknown.

They allow you to create infinitely scalable graphics right within your web browser. It uses the same concepts as HTML and CSS, and is therefore a small language you can learn in no time.

Creating them dynamically is of course flexible and powerful. But if you just want fixed vector graphics, e.g. for icons, I advice using software for this and exporting to .svg when done.