Apple today released a new update for Safari Technology Preview, the experimental browser that debuted on March 30. Release 2 includes a long list of feature tweaks and updates to JavaScript, CSS, Web APIs, Web Inspector, Accessibility, Rendering, Media, and Networking, plus bug fixes. Release notes are available on Apple's Safari Technology Preview website.
The Safari Technology Preview update is available through the Mac App Store to anyone who has downloaded the browser.
Apple's goal with Safari Technology Preview is to gather feedback from developers and users on its browser development ahead of launch. Safari Technology Preview can be run side-by-side with the existing Safari browser and while aimed at developers, it does not require a developer account to download.
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Why do they need to do this? Well the features of ES6/7 are not completely native to current browsers (namely Chrome, Firefox and Safari)... Well, I say that... actually the big browsers at the moment do have support for the new ES features but not to the extent where developers can feel free to run native ES6/7. At the moment, developers have to transpile code written in the newest, fastest, latest spec of JavaScript to an older version that has a much better coverage on all browsers. Safari (along with many other browsers) didn't have any of the new JS features so developers couldn't use any of said features unless they were prepared to transpile their code. Chrome, Firefox and Microsoft Edge are all slowly updating but Safari hasn't. The Technology Preview finally both evidence that Apple will definitely include ES6 in the next version of Safari and it's also a way for developers to start testing out their code on an official development branch of Safari.
So just like Chrome has stable Chrome, beta Chrome and Chrome Canary, Safari now has stable Safari, Safari Technology Preview and WebKit.
Excuse me if my explanation was off but that's the way I see it. I'm not Apple so I don't have the official story.
I have about >20 open tabs and have no problems.