The style declarations which apply to the text below are:

.zero {background-color: silver; border-top-width: 0;}
.one {border-top-width: 25px; border-style: solid;}
.two {border-top-width: thick; border-style: solid;}
.three {border-top-width: medium; border-style: solid;}
.four {border-top-width: thin; border-style: solid;}
.five {border-top-width: 25px;}

(These will only work if border-style is supported.)

This element has a class of zero.

This paragraph should have a top border width of 25 pixels.

This paragraph should have a thick top border width.

This paragraph should have a medium top border width.

This paragraph should have a thin top border width.

This paragraph should have no border and no extra "padding" on its top side, as no border-style was set.