Safari had this feature a few years back but appear to have removed it sometime about 3 years ago. It was great, but also removed all ads from the page, so it probably wasn't popular with online news sites.
I use it regularly in Safari. I also have an HN-app on my iOS-devices giving me a similar "reader view" and removing fluff from the websites. I find it really pleasing when just reading articles (I've often caught myself going into devtools to remove navigation bars, sidebars, etc from websites when reading longer articles since they distract me).
https://support.mozilla.org/en-US/kb/firefox-reader-view-clu...