Grouping selectors and rules

From

body { color: purple }
body { background-color: #d8da3d }

To (Rules can be combined : We have set two properties, so we could have made two separate rules)

body { color: purple;
       background-color: #d8da3d }

From

H1 { font-weight: bold }
H2 { font-weight: bold }
H3 { font-weight: bold }

To (same Declaration)

H1, H2, H3 { font-style: bold }

Source : http://www.w3.org/Style/Examples/011/firstcss and http://www.w3.org/Style/LieBos2e/enter/