The "object" tag below has a "type" attribute set to "application/json".
Still, Firefox sends an "HTTP Accept: text/html…" header, while it should actually ask for an "application/json".
This is problematic if the server, like https://isometry.reinom.com, uses the HTTP Accept header to return a properly formatted response. Since Firefox sends a wrong HTTP Accept, then the server returns an HTML response instead of a JSON one.
The use case of this could be embedding an SVG in a webpage: if the server received the proper "image/svg+xml" header, then it would return an SVG, but since it receive a "text/html" header, then it returns an HTML page, which is not what the "object" tag expects.

Setting no "type" attribute:

Setting "type" to "application/json":

Setting "type" to "image/svg+xml":

Check the other showcases