Have you ever Googled something, clicked on the first link on the results page and been really impressed by how fast the site loaded?
If so, it’s possible that you’ve seen prerendering in action.
If Google is confident enough that the top result in its list is the one you want, it will sometimes render the entire page in the background. This isn’t a new thing – it’s been doing it since 2011, with Bing following suit in 2013.
We tried searching for “youtube” in google.co.uk, using Chrome. Unsurprisingly, Google was pretty sure we wanted youtube.com, so it prerendered the page in a background tab, invisible to the end user.
How can you tell that a page has been prerendered if it’s invisible?
One way to find out is to launch the Task Manager. You can do this by right-clicking on the bar at the top of your browser window and selecting “Task Manager” from the menu.
You’ll see a window just like the one below, with the title of the prerendered page helpfully prefixed with “Prerender:”.
Figure 1 – Prerendering shown in the Chrome Task Manager
Alternatively, look at chrome://net-internals/#prerender. As well as showing the status of any active prerender pages, this will also give you a history.
Figure 2 – Watch prerendering as it happens by looking at chrome://net-internals/#prerender
You can even look at the DNS lookups that have been preresolved during prerendering by navigating to chrome://dns:
Figure 3 – Preresolved DNS lookups in Chrome
How often does Google do this?
Sometimes, you can be pretty sure that Google will prerender the page. For example, if you search for a company name, it will probably prerender the company’s home URL. To check this, we Googled the names of the UK’s top 50 retailers and found that 47 home pages were prerendered.
Chrome will also use your browsing history to work out which (if any) pages to prerender when you type something into the omnibox (see this article by Steve Souders for more on this: http://www.stevesouders.com/blog/2014/04/30/unexpected-prerender-in-chrome/).
What about generic search terms?
As far as we can tell, this seems to vary quite a bit from one search term to the next. Sometimes, the top result will be prerendered and sometimes it won’t.
This information could be quite useful for marketers. If your home page is prerendered when people search for a particular key phrase, this tells you something about how far ahead of the competition you are. You also know that your competitors are going to have to work a little harder to catch up with you: other things being equal, the other sites on the results page will appear to load more slowly than yours.
Conversely, perhaps you’re hoping to replace a rival at the top of the search results page. If your competitor’s page is prerendered, you know you need to prioritise your own site’s performance in order to negate the speed advantage that prerendering gives them.
Do prerendered pages show up in analytics?
They shouldn’t do – at least if you’re using Google Analytics. The page visibility API (read more here) allows prerendered pages to be filtered out of the results. Since prerendering has been around for a while now, it seems reasonable to suppose that most other analytics providers and third-party tag solutions take it into account too.
If you’re interested in tracking how often your pages are getting prerendered, you can also use the page visibility API for this.
Is prerendering top search results a good thing?
Arguably, the practice helps to tip the scales in favour of the big names, working to preserve the status quo, as well as rewarding poor performing sites disproportionately.
However, this probably overstates the benefits that prerendering offers. It’s certainly worth knowing whether and when Google or Bing is giving your site that little extra boost, but it’s important to remember that it’s only one of large number of factors that affect your site’s success in search results.
And if you’re worried about your competitors having an unfair advantage, you can go a long way towards removing it by optimising your own site.