Hello, this was me; author of JMAP and lead architect of Fastmail's web UI. The advice was intended to be "render it as plain text" AND then put it in a <pre> to preserve white space. Turns out tweets are difficult to put technical advice into, especially when you are not sure of the context the person you are talking to will bring with (so to me "render as plain text" means insert as a text node, not as innerHTML, but looking back I can see that this may not mean anything to someone with a different experience of building web apps). My apologies for the confusion.