Tabs should be used for indentation.
In general, properties further towards the top should affect layout, while properties towards the bottom should affect appearance.
Specifically, the most common properties should be in this order:
.class {
position
display
flex /* etc. */
top
bottom
left
right
z-index
width /* also min-, max- */
height /* also min-, max- */
padding
border
margin
font /* etc. */
text-align
background /* etc. */
color
}