Why do large Web sites write CSS directly on the page?

In the view of Tencent, Sina, the layout of these large sites, found a problem, why they will put the CSS directly on the page, rather than separate as a CSS file link in.

This is a kind of optimization scheme to deal with super large flow.

To improve speed, their external connection files are generally stored on a separate server, such as img.****.com, if the server is hung up, the entire front page is hung off. On the page, more secure. When a user accesses a page, it does not result in a bare page because the CSS does not load.

Does this increase the cost of maintenance? If written on the page, it is not easy to modify and manage.

He could have been a separate CSS file. Include a php file in the header with include, and the PHP file is responsible for generating the CSS file.

You are actually maintaining a separate style sheet. Just put it together when it's up. Maintenance costs are not increased.

Using PHP to call CSS to the page, the server also has the cost?

Slow down, the homepage is cached as a static page.