From Chris McCord examples on github messages on firefox…
One or more documents in this page is in Quirks Mode, which will render the affected document(s) with quirks incompatible with the current HTML and CSS specifications.
Quirks Mode exists mostly due to historical reasons. If this is not intentional, you can add or modify the DOCTYPE to be `<!DOCTYPE html>` to render the page in No Quirks Mode.
Document in the DOM tree Mode URL
document Quirks Mode http://localhost:4000/phoenix/live_reload/frame
Cookie “_demo_key” will be soon rejected because it has the “SameSite” attribute set to “None” or an invalid value, without the “secure” attribute. To know more about the “SameSite“ attribute, read https://developer.mozilla.org/docs/Web/HTTP/Headers/Set-Cookie/SameSite
Have you tried 127.0.0.1 instead of localhost? On my Windows computers localhost has problems with WebSocket connections on Chrome. WebSocket connections take very long time to connect. But 127.0.0.1 works fine.
Exactly. Can you check source code from browser debug tool? Because it looks like your JS is not executed on that machine. Which browser do you use? Do you use some extensions? Ad blockers? What about log from your terminal/console when you start Phoenix app, usually webpack “compile” your assets and generate main js file. What do you use for assets? esbuild? node? webpack?