Svg (Scalable Vector Graphics) is an XML-based vector image format.
The advantage is that you can modify image size without losing quality and detail.
Scaling the vector image preserves the shapes. This can be rendered at any resolution.
How to use SVG with Html (Non-exhaustive list) :
▶ Here are some nice tools for Svg pictures
To create a font file (example.ttf) use Icomoon
To edit svg online ... you can use this Svg Editor
A vector graphics editor software : Inkscape
For a better conversion result, prefer pictures with solid background.
Picsvg is also an easy way to convert icon to svg.
It's a beta version : if you get errors, send feedback.