Viewport-percentage lengths defined a length relatively to the size of viewport, that is the visible portion of the document. Only Gecko-based browsers are updating the viewport values dynamically, when
the size of the viewport is modified (by modifying the size of the window on a desktop computer or by turning the device on a phone or a tablet).
1vh - 1% of height
1vw - 1% of width
vmin and vmax - minimum and maximum value from width and height.
<meta name="viewport" content="width=device-width, initial-scale=1"/>
Browser support: here.
Source: Mozilla Developer Network