HTML and CSS Reference
In-Depth Information
Making your site faster
If your pages use a lot of resources such as images, then perhaps it would be wise
to prefetch those resources so your page loads faster. DNS prefetching would be a
way to do that.
DNS prefetching
DNS prefetching informs the browser of resources on other domain names that are
referred to within the page early on during the page load, so it can do the DNS resol-
ution of these domain names.
A browser has to look up a domain name on a Domain Name Server ( DNS ) to figure
out where it is located on the Internet. Sometimes, it has to go through multiple layers
of Domain Name Servers and it could be very slow and is not always consistent. By
using DNS prefetching, even before a user clicks a link, or loads a resource, the DNS
resolution for that particular domain name is done and the resource can be fetched
much faster.
Google states that this saves about 200 milliseconds on a resource hosted on an ex-
ternal domain name.
If you host your assets on a Content Delivery Network ( CDN ) like Amazon's S3 or
even if you refer to Google's API or Microsoft's API CDN, it would be faster to get
these files when they are prefetched.
DNS prefetching is invoked by writing the following code within the head tag of a
HTML file:
<link rel="dns-prefetch"
href="//image.cdn.url.example.com">
Browsers that understand prefetching would immediately start attempting to resolve
the DNS for the link within the href attribute. The following is how it would look for
Amazon S3:
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