We always strive to keep up with a fairly aggressive browser target. More is written here, What is Discourse? | Discourse - Civilized Discussion
At the moment we officially no longer support iOS 9. What that means is that we can lean on more browser features with less messy poly fills.
As to what will happen here:
@j.jaffeux will be investigating this.
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If there is a trivial fix here, we will get it sorted
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If anything is looking complicated we will fix it so we properly display a “browser not compatible” message on the page.