From Nicole's presentation at the CSS Summit. This is brand new research regarding efficient CSS selector design. Practicing the rules outlined here will make your CSS lean, your site fast, and your maintenance minimal. A beautiful combination for people concerned with building performance into their sites.

1. TAMING SELECTORS
sanity for our stylesheets

2. SIZE OF THE CSS FILE AND
IMPLIED HTTP REQUESTS
are the biggest factor for CSS performance

3. REFLOWS AND
RENDERING TIME
are (much!) less important

4. DUPLICATION IS WORSE
THAN STALE RULES
because we have tools to deal with the later

5. DEFINE DEFAULT VALUES
Don’t repeat this code in every object

6. #weatherModule h3{color:red;}
#tabs h3{color:blue}
DEFINE DEFAULT VALUES
Don’t repeat this code in every object

7. X
#weatherModule h3{color:red;}
#tabs h3{color:blue}
DEFINE DEFAULT VALUES
Don’t repeat this code in every object

8. X
#weatherModule h3{color:red;}
#tabs h3{color:blue}
DEFINE DEFAULT VALUES
Don’t repeat this code in every object
h1, .h1{...}
h2, .h2{...}
h3, .h3{...}
h4, .h4{...}
h5, .h5{...}
h6, .h6{...}

9. DEFINE STRUCTURE IN A
SEPARATE CLASS
Don’t repeat this code in every object
block
inner
hd
bd
ft

10. div.error{...}
STYLE CLASSES
rather than elements

11. X
div.error{...}
STYLE CLASSES
rather than elements
.error{ most of the code goes here }

12. X
div.error{...}
STYLE CLASSES
rather than elements
.error{ most of the code goes here }
div.error{ }
p.error{ exceptions only }
em.error{ }

13. div{...}
ul{...}
p{..}
AVOID STYLING ELEMENTS
unless defining defaults

14. div{...}
X
ul{...}
p{..}
AVOID STYLING ELEMENTS
unless defining defaults
.error{...}
.section{...}
.products{...}

15. .myModule .inner b{...}
.myModule2 b {...}
GIVE RULES THE
SAME STRENGTH
Use cascade order to overwrite previous rules

16. X
.myModule .inner b{...}
.myModule2 b {...}
GIVE RULES THE
SAME STRENGTH
Use cascade order to overwrite previous rules
.myModule b{...}
.myModule2 b {...}

17. .sidebar ul{...}
.header ul {...}
AVOID SPECIFYING
LOCATION
Apply classes to the object you wish to change

18. X
.sidebar ul{...}
.header ul {...}
AVOID SPECIFYING
LOCATION
Apply classes to the object you wish to change
.mainNav{...}
.subNav {...}

19. AVOID OVERLY SPECIFIC
CLASSES
they’re all semantic, but limiting

20. AVOID OVERLY SPECIFIC
CLASSES
they’re all semantic, but limiting
.vehicle{...}
.motorcycle{...}
.ducati{...}
.ducatiMonster620{...}
.nicolesDucatiMonster620{...}

21. AVOID OVERLY SPECIFIC
CLASSES
they’re all semantic, but limiting
.vehicle{...}
.motorcycle{...}
.ducati{...}
X
.ducatiMonster620{...}
.nicolesDucatiMonster620{...}

22. #myUniqueIdentifier{...}
#myUniqueIdentifier2{...}
AVOID SINGLETONS
ids can only be used once in any given page
Source: Chris Griego

23. X
#myUniqueIdentifier{...}
#myUniqueIdentifier2{...}
AVOID SINGLETONS
ids can only be used once in any given page
Source: Chris Griego

24. Media
Subheading
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna
aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis
aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat
cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
USE MIXINS
to avoid repeating code
Media Extended
Subheading
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna
aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis
aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat
cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

25. .inner{...} .weatherMod .inner{...}
.tr{...} .weatherMod .tr{...}
.bl{...} .weatherMod .bl{...}
ENCAPSULATION
don’t access sub-nodes of an object directly
inner

26. .inner{...}
X
.weatherMod .inner{...}
.tr{...} .weatherMod .tr{...}
.bl{...} .weatherMod .bl{...}
ENCAPSULATION
don’t access sub-nodes of an object directly
inner