1.Whereas HTML has tags,CSS has selectors.
2.Selector{
properties:value;
properties2:value2;
}
3.Lengths & Percentages
px:unit for pixels
em:unit for the calculated size of a font;N times the current size
pt:unit for points ,printed media
%:
pc-picas,cm-centimeters,mm-millimeters,in-inches
zero value,no need to state a unit.