If developers made sure their sites worked in Safari we wouldn't have submissions to HN for sites that only work in Chrome with "oh yeah it doesn't work in Firefox or Safari because I didn't test it".
The number of developers who do their work on Chrome and call it a job done and never test in another browser, far outweight the number of developers who do test cross-browser.
The reason people tested in IE11 and less was because they were forced to. They liked Chrome but the users were using the shitty browsers, and older OS didn't yet support Edge.
Now the majority use Chrome and the devs no longer care.
We need more competition in the browser space, Safari and Firefox provide that. And neither are holding web development back.
Developers are.