Current latest version of jQuery is 1.11 or 2.1
-Each new version of jQuery optimizes performances. Do not keep an old version of jQuery. Updating can break a few things, but it is generally quite easy to fix them up, so don't hesitate.
+Current latest versions of jQuery are 1.11 (with support for old IE versions) and 2.1 (without).
+Each new version of jQuery optimizes performances. Do not keep an old version of jQuery. Updating can sometime break a few things, but it is generally quite easy to fix them up, so don't hesitate.
jQuery is a heavy library. You should **never** load jQuery more than one on the same page.
Complex selectors are CSS selectors with 4 or more expressions, like "header ul li .foo".
+They are adding more work for the browser, and this could be avoided by simplifying selectors.
This is when two or more selectors are strictly identical and should be merged.
Very easy to fix.
Such as: expression( document.body.clientWidth > 600 ? "600px" : "auto" )
+This is a bad practice as it slows down browsers. There are some simpler CSS3 methods for doing this.
It can be usefull, but use it only as a last resort. It is a bad practice because it overrides the normal cascading logic. The more you use !important, the more you need it again to over-override. This conducts to a poor maintainability.
What browser do you need to support? Once you got the answer, take a look at these old rules that pollute your CSS code and remove them.
+IE6: +
+
IE7: +
+
IE9: +
Many property prefixes such as -moz- or -webkit- are not needed anymore, or by very few people. You can remove them or replace them with the non-prefixed version. This will help reducing your stylesheets weight.
Universal selectors are the most expensive CSS selectors.
+More informatons here.
This is one way to remove complexity from a CSS rule. Generally, when "body" is specified in a rule it can be removed, because every element is inside the body.
This is one of the most important performance rule. Every request is slowing down the page loading.
+There are several technics to reduce their number: +
404 errors are never cached, so each time a page ask for it, it hits se server. Even if it is behind a CDN or a reverse-proxy cache.
This counts the number of requests not keeping the connection alive (specifying "Connection: close" in the response headers). It is only counting a request if it is followed by another request on the same domain.
+This is slowing down the next request, because the brower needs to open a new connection to the server, which means a additional round-trip.
+Correct the problem by setting a Keep-Alive header on the guilty server.
This only happens when the asset has no cache and is requested more than once on the same page. Be very careful about it.
Counts responses with caching disabled (max-age=0)
+Fix immediatly if on static assets.
Responses with no caching header sent (either Cache-Control or Expires).
+Every request should have a cache time specified. If you really don't want cache, specify "max-age=0", otherwise some browsers will try to cache.
Responses with too short caching time (less than a week).
+The longer you cache, the better. Add versionning to your static assets, if it's not already done, and set their cache time to one year.
For each domain met, the browser needs to make a DNS look-up, which is slow. Avoid having to many different domains and the page should render faster.