Border

border of elements

span elements create 3 rectangular boxes

border-width: thin, thick and medium.
border-style: solid, dashes dotted etc

tip - with our border width, specifying using keywords the actual thickness is left up to the browser - checking in diff browser, may be diff and cause some issues

Integer values... 10 px - or 10ems - etc can be used for border width.

width and height properties of a box
just put
width:30px
height:30px

you can change each border side
border-top-width
right left bottom etc instead of just saying thick
you can also specify the style for each side - just add style where width is.

border colors

overall and individual border colours
border-color:green; - individual
or
border-top:red; - individual
border-bottom:green;