These tips are based on common knowledge and experimentation.
An optimized web page not only provides for a more responsive site for your visitors but also reduces the load on your web servers and internet connection. This can be crucial for high volume sites or sites which have a spike in traffic due to unusual circumstances such as breaking news stories.
Optimizing page load performance is not just for content which will be viewed by narrowband dial-up or mobile device visitors. It is just as important for broadband content and can lead to dramatic improvements even for your visitors with the fastest connections.
Page weight is by far the most important factor in page-load performance.
Reducing page weight through the elimination of unnecessary whitespace and comments, commonly known as minimization, and by moving inline script and CSS into external files, can improve download performance with minimal need for other changes in the page structure.
Reducing the number of files referenced in a web page lowers the number of HTTP connections required to download a page, thereby reducing the time for these requests to be sent, and for their responses to be received.
Depending on a browser's cache settings, it may send a request with the
If-Modified-Since header for each referenced file, asking whether the file has been modified since the last time it was downloaded. Too much time spent querying the last modified time of the referenced files can delay the initial display of the web page, since the browser must check the modification time for each of these files, before rendering the page.
If you use background images a lot in your CSS, you can reduce the number of HTTP lookups needed by combining the images into one, known as an image sprite. Then you just apply the same image each time you need it for a background and adjust the x/y coordinates appropriately. This technique works best with elements that will have limited dimensions, and will not work for every use of a background image. However, the fewer HTTP requests and single image caching can help reduce page-load time.
For the purposes of this article, a CDN is a means to reduce the physical distance between your server and your visitor. As the distance between your server origin and visitor increases, the load times will increase. Suppose your website server is located in the United States and it has a visitor from India; the page load time will be much higher for the Indian visitor compared to a visitor from the US.
A CDN is a geographically distributed network of servers that work together to shorten the distance between the user and your website. CDNs store cached versions of your website and serve them to visitors via the network node closest to the user, thereby reducing latency.
This may not always be practical; however, you should always take care to use only the minimum necessary number of different domains in your pages.
Make sure that any content that can be cached, is cached, and with appropriate expiration times.
In particular, pay attention to the
Last-Modified header. It allows for efficient page caching; by means of this header, information is conveyed to the user agent about the file it wants to load, such as when it was last modified. Most web servers automatically append the
Last-Modified header to static pages (e.g.
.css), based on the last-modified date stored in the file system. With dynamic pages (e.g.
.aspx), this, of course, can't be done, and the header is not sent.
So, in particular, for pages which are generated dynamically, a little research on this subject is beneficial. It can be somewhat involved, but it will save a lot in page requests on pages which would normally not be cacheable.
Inline scripts can be expensive for page loading since the parser must assume that an inline script could modify the page structure while parsing is in progress. Reducing the use of inline scripts in general, and reducing the use of
document.write() to output content in particular, can improve overall page loading. Use DOM APIs to manipulate page content, rather than
Use of modern CSS reduces the amount of markup, can reduce the need for (spacer) images, in terms of layout, and can very often replace images of stylized text — that "cost" much more than the equivalent text-and-CSS.
Using valid markup has other advantages. First, browsers will have no need to perform error-correction when parsing the HTML (this is aside from the philosophical issue of whether to allow format variation in user input and then programmatically "correct" or normalize it; or whether, instead, to enforce a strict, no-tolerance input format).
Moreover, valid markup allows for the free use of other tools that can pre-process your web pages. For example, HTML Tidy can remove whitespace and optional ending tags; however, it will refuse to run on a page with serious markup errors.
Tables are still considered valid markup but should be used for displaying tabular data. To help the browser render your page quicker, you should avoid nesting your tables.
Rather than deeply nesting tables as in:
use non-nested tables or divs as in
SVG produced by most drawing applications often contains unnecessary metadata which can be removed. Configure your servers, apply gzip compression for SVG assets.
If the browser can immediately determine the height and/or width of your images and tables, it will be able to display a web page without having to reflow the content. This not only speeds the display of the page but prevents annoying changes in a page's layout when the page completes loading. For this reason,
width should be specified for images, whenever possible.
Tables should use the CSS selector: property combination:
By default, images are loaded eagerly; that is, the image is fetched and rendered as soon as it's processed in the HTML. All eagerly loaded images are rendered before the window's
load event is sent. Switching to lazy loading of images tells the browser to hold off on loading images until they're about to be needed to draw the visual viewport.
To mark an image for lazy loading, specify its
loading attribute with a value of
lazy. With this set, the image will only be loaded when it's needed.
<img src="./images/footerlogo.jpg" loading="lazy" alt="MDN logo" />
Note that lazily-loaded images may not be available when the
load event is fired. You can determine if a given image is loaded by checking to see if the value of its Boolean
complete property is
To achieve the greatest improvements in page design, make sure that reasonable user-agent requirements are specified for projects. Do not require your content to appear pixel-perfect in all browsers, especially not in down-version browsers.
Ideally, your basic minimum requirements should be based on the consideration of modern browsers that support the relevant standards. This can include recent versions of Firefox, Google Chrome, Opera, and Safari.
Note, however, that many of the tips listed in this article are common-sense techniques which apply to any user agent, and can be applied to any web page, regardless of browser-support requirements.
CSS files required for page appearance. Minimize the number of files for performance while keeping unrelated CSS in separate files for maintenance.
User visible page content in small chunks (
) that can be displayed without waiting for the full page to download.