- Your user requests your web app
- Has the page loaded yet?
- “All your users are non-JS while they're downloading your JS” — Jake Archibald
- How many times have you had a mobile browser hang forever loading a page and then load it instantly when you refresh?
- Because loads of them still do.
- Sky accidentally block jQuery, Comcast insert ads into your script, and if you've never experienced this before, drive to an airport and use their wifi.
- People still do.
- Do they have addons or plugins installed which inject script or alter the DOM in ways you didn't anticipate?
- There are thousands of browser extensions. Are you sure none interfere with your JS?
- Is the CDN up?
- CDNs are good at staying up (that’s what being a CDN is) but a minute downtime a month will still hit users who browse in that minute.
- Check Can I Use for browser usage figures.
Is all the above true?