What is right way to add CSS to some pages?

When you have CSS aggregation turned on (and you should), Drupal has to combine all the stylesheets together and save it in a new file.

But when you conditionally include an additional stylesheet via drupal_add_css(), you force Drupal to rebuild the aggregated stylesheet for that page. Which is 1.) a significant performance hit for Drupal and, worse, 2.) a big performance hit for your website’s users because they have to download a new really large aggregated stylesheet. :-p

So, the way to avoid that is to:

1. non-conditionally include your stylesheets by adding them to your theme’s .info file,
2. add some conditional code to your theme’s mysubtheme_preprocess_page function (in template.php) that adds a new class to the $body_classes variable:
   
   <?php
function MYTHEMENAME_preprocess_page(&$vars, $hook) {
  if ($condition) {
     $vars['body_classes'] .= ' conditional-class-1';
  }
}
?>
3. prefix all the rules in your "conditional" stylesheet with the class that you conditionally added in step 2.

So now the aggregated stylesheet remains the same on each page request, but the styles that get applied depend on the body classes.

JohnAlbin THANX A LOT!
That's what I call right USEFUL clever answer post!
great :)

^{(more stuff like this here :)}

Automatically closed -- issue fixed for 2 weeks with no activity.