HTTP cookies are widely used for authentication and personalization. Cookies are stored in HTTP request headers, transmitted between browsers and servers. It's very important for us to keep the size of cookies as low as possible to minimize the impact on the user's response time, especially for a large-scaled system.

Use localStorage for storing the data that your server doesn't care about

For example, if you happened to be the developer of GitHub, and want to make sure when a user is editing an issue page, the content in the text editor won't disappear if he refreshes the page. In this case, you should use localStorage to store temporary data in the browser. Don't put this kind of stuff in cookies ever.

Host static resource from a different domain to avoid unnecessary cookie traffic

It's a bad practice to host your static resources, i.e. assets, images and other content, under the same domain as your main site. If you really did, every HTTP request users made to the static resources will carry a copy of the cookies of under your main domain, which is totally a waste of the bandwidth.

For instance, if you happened to develop github.com again and need to find a domain name to put the logo image of the site, you should avoid putting it either at github.com/images/logo.gif or at images.github.com/logo.gif. If users visit either of the above URLs, they will put a copy of the cookies of github.com inside the HTTP header. A good way to deal with this is to get a new domain for serving static files. For example, GitHub uses githubusercontent.com to serve user uploaded images.

Also, it is notable that browsers have maximum concurrent get request to the same domain, so serving static resources from a different domain also improves the loading speed of the web page more or less.

Keep your Cookies short

To keep the cookies short, both the key and value should be as short as possible. For example, it's a good practice to use 0 and 1 for boolean values rather than false and true, and use u as the key for storing username rather than use username, which is too long.