Setting the display property to inline is an alternative to setting the float property to left.
li{
display: inline;
margin-right: 20px;
list-style: none;
}
Using float left is a popular way to make a list horizontal. It tends to have less problems then the alternative, which is using display: inline.
li{
float: left;
margin-right: 20px;
list-style: none;
}