Web page HTML/CSS/JavaScript file size

I found this article (Some Guidelines for Determining Web Page and File Size) today which talks about the average size of HTML and other files on the web. According the article (and I’m not clear how they got their data) the average HTML file is 25k, JPEG 11.9k, GIF 2.9k, PNG 14.5k, SWF 32k, external scripts 11.2k and external CSS 17k with the average total size of a web page being 130k. Interesting stuff. Particularly that scripts are typically 11.2k given that jQuery is 90k.

I’m really struggling with a design decision at the moment, being that I’m not sure whether it’s better to embed CSS/JavaScript content or to link it. The thing is that if you link it then the client has to send extra HTTP requests (at least two) to get the content, which is overhead and takes time. The thing is, if your users are returning customers then they might already have the linked files in their cache, meaning they don’t need to send extra HTTP requests, or if they do maybe those requests won’t need to return content. But then maybe a browser will cache a file when it shouldn’t (this can be avoided with good design), or maybe the user’s connection will fail while loading the linked files and they’ll see an unstyled page in their browser.

So many pros and cons, and it’s all hypothetical… what I really need is data. Anyway, I don’t have data, nor do I really have the tools to get it. So given that I have to fly in the dark, here’s my plan:

When I’m processing a request for a user who doesn’t have a browser cookie set I will embed CSS and JavaScript in the HTML. This is because if their browser cookie isn’t set then this is their first request to my web-site, maybe ever, or maybe just in a while. Either way, it’s probably safe to assume they’re a first-time visitor so they won’t have any content in their cache and they’d need to send additional requests for linked files. So I can save those additional requests and hopefully make my web pages load faster for users who are probably one-off visitors.

But for regular users having to download the same content over and over in every request gets tired fast. The linked files can be about half the size of the page, so embedding doubles the size of each transfer. When I’m processing a request if the user’s browser cookie is already set then I’ll assume they’re a regular visitor and link my JavaScript files rather than embedding them. I’ll still embed CSS content though, because my CSS content is relatively small and I want to avoid errors where the page loads but the styles don’t.

Then I’ll make the system configurable so users can change their link/embed settings for CSS and JavaScript if they’re not happy with the defaults. Regular power users can use this feature to turn on linking for all content so pages load as fast as possible for them.

2 thoughts on “Web page HTML/CSS/JavaScript file size

  1. I think requests are generally done in parallel though – so the browser will start parsing your page before it’s completely loaded. If there are any external resource links in there, it’ll start loading them immediately.

    You really want CSS to load first, so whack the link elements in head, and put JS down the bottom.

    This explains it better than I can:

  2. Yep, knew that dude. I put my CSS in head, and the scripts as the last content in body. The browser keeps two (or is it four?) HTTP connections open to the server at a time and resources are downloaded through those while the HTML is parsed (and scripts executed). As I mentioned in a later blog post you should embed rather than link to small CSS. In the web-framework I’m writing at the moment all CSS/JavaScript/image content can be either linked or embedded and there are some smarts to choose something optimal(ish) for you if you don’t specify your own preferences. Basically regular users should link everything (so that it’s cached) and one-time users should embed everything (so that they don’t have to send multiple HTTP requests).

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