Unlike Chrome and Firefox, Opera 23 doesn't suffer invisible popups. It does use the same Webkit engine and run as multiple processes, and it's more difficult to shut down. It also lacks a home button, instead relying solely on a launch page of oft-used sites (Speed Dial).
Speed Dial is great if you work off of multiple sites, but it adds another step for those who work from a single page. My minor complaint: There's no native feature for emailing a link to a page. You must install an extension for this functionality.
Despite the list of minor complaints, Opera is currently my main work browser because it's nearly as fast as Chrome, but easier to configure.
Safari 5.1.17 (WebKit 534)
Though king on the Mac, Safari is probably the least popular of the top browsers under Windows — a bit sad as it's competent, easy to use, and light on the memory profile. Apple apparently ceased supporting Windows as of version 5.1.17, but we tested it anyway because of its thrifty use of memory.
And 'thrifty' is understating the case. It spawns only a single process and in my nine-tab hands-on test, required only 115MB of memory to display it. What happened between the WebKit 534 that Safari uses, and WebKit 537, the current choice of Chrome and Opera is quite amazing. The latter offers way more speed but sucks up a lot of resources.
Safari could be very handy on older systems with only 512MB or 1GB of memory. It has all the standard features found in the others, so you're not giving away anything.
If you want the fastest browser for rendering HTML — by far the majority of the actions that a browser performs — you have a choice between Chrome and Opera. They both spawn a lot of processes and memory to accomplish it. Of the two, I'd pick Opera for overall easier configuration.
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