Failing to load is not a problem. Failing to execute is.
Usually because the web dev have used some new Javascript feature only $latest JS engines support. HTML and CSS if they're there they're there. Sure, there's caniuse for HTML and CSS. But they only have to load. The text/images/etc will be there. JS both has to be loaded and executed. If the later doesn't happen just right then the text and other multi-media won't be there. It's a very big difference.
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If your HTML or CSS fails to load, you're going to have a hard time too. Web pages have many critical resources.
Failing to load is not a problem. Failing to execute is.
Usually because the web dev have used some new Javascript feature only $latest JS engines support. HTML and CSS if they're there they're there. Sure, there's caniuse for HTML and CSS. But they only have to load. The text/images/etc will be there. JS both has to be loaded and executed. If the later doesn't happen just right then the text and other multi-media won't be there. It's a very big difference.