参考：
http://developer.yahoo.com/performance/rules.html#num_http
80% of the
end-user response time is spent on the front-end
. Most
of this time is tied up in downloading all the components in the page: images,
stylesheets, scripts, Flash, etc. Reducing the number of components in turn reduces
the number of HTTP requests required to render the page. This is the key to
faster pages.
简化页面很重要，但是往往不可行，如何在保持页面内容丰富多彩的前提下保持快速的响应时间，这是优化的重点！
One way to reduce the number of components in the page is
to simplify the page's design. But is there a way to build pages with richer
content while also achieving fast response times? Here are some techniques for
reducing the number of HTTP requests, while still supporting rich page designs.
Combined
files
are a way to reduce the number of HTTP
requests by combining all scripts into a single script, and similarly combining
all CSS into a single stylesheet. Combining files is more challenging when the
scripts and stylesheets vary from page to page, but making this part of your
release process improves response times.
CSS Sprites
are the preferred method for reducing the number of image requests. Combine
your background images into a single image and use the CSS background-image
and background-position
properties to display the desired image
segment.
Image maps
combine multiple images into a single image. The overall size is about the
same, but reducing the number of HTTP requests speeds up the page. Image maps
only work if the images are contiguous in the page, such as a navigation bar.
Defining the coordinates of image maps can be tedious and error prone. Using
image maps for navigation is not accessible too, so it's not recommended.
Inline
images
use the data:
URL scheme
to
embed the image data in the actual page. This can increase the size of your
HTML document. Combining inline images into your (cached) stylesheets is a way
to reduce HTTP requests and avoid increasing the size of your pages. Inline
images are not yet supported across all major browsers.
Reducing the number of HTTP requests in your page is the
place to start. This is the most important guideline for improving performance
for first time visitors. As described in Tenni Theurer's blog post Browser
Cache Usage - Exposed!
, 40-60%
of daily visitors to your site come in with an empty cache. Making your page
fast for these first time visitors is key to a better user experience
.
每天的访问量中，有
40~60%
的网站访问者是第一次访问，他们的浏览器中并没有你这个网站的缓存文件。