Using PX when it's not Needed

Bad

.class-name {
    padding: 10px 0px;
}

Good

.class-name {
    padding: 10px 0;
}

Not Using Shorthand Properties

Bad

.class-name {
    margin-top: 10px;
    margin-bottom: 10px;
    margin-left: 10px;
    margin-right: 10px;
}

Good

.class-name {
    margin: 10px;
}

Using Font Fallback

Bad

.class-name {
    font-family:Helvetica; 
}

Good

.class-name {
    font-family:Arial, Helvetica, sans-serif; 
}

Repeating the Same code

Bad

.class-name {
    margin: 10px;
    padding: 10px;
}

.other-class-name {
    margin: 10px;
    padding: 10px;
}

Good

.class-name, .other-class-name {
    margin: 10px;
    padding: 10px;
}

Over Qualifying Selectors

Bad

.navbar ul.menu li a {
   color: #000000
}

Good

.menu a {
    color: #000000
}

Using Color Names

Bad

.class-name {
    color: black
}

Good

.class-name  {
    color: #000000
}