Right, but your original post mentioned https://.static.observableusercontent.com. 
Looks like Discourse interpreted <my username> as an HTML tag and dropped it, because it wasn’t marked up as code.
Are you using a proxy for web access? Could there be something sitting inbetween your network connection that is modifying response headers?
I’d recommend to inspect the network request: Open your browser’s developer tools, then go to the Network tab and look for a request for d3.min.js. Select that request and review the response headers. They should roughly look like this:
