I think you will find that Discourse tends to be actively developed and uses leading-edge browser features. most sites you visit on the 'net (BoingBoing included) tend to try to cater to the lowest-common-denominator and so many advanced features that would result in a more responsive, less resource-intensive are adopted extremely slowly because sites have no choice but to cater to clients that do not support them (and why waste time and money developing technologies for a site if only a subset of users can benefit?).
Discourse on the other hand has no issue deprecating browser versions as soon as their traffic drops sufficiently across the web, as part of their quest to be a low-resource, progressive-web-application.
As an example, Discourse already dropped support for IE: