2017-11-21 UTC
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Zegnat alt="" is probably right, upon scrolling through some quick resources. As any description of the person’s display picture would offer redundant information to the text right next to it (their name), and for redundancy reasons alt="" is apparently fine. See e.g. https://www.w3.org/WAI/tutorials/images/decision-tree/ and https://webaim.org/techniques