I don't think it's so much technical difficulty as much as it is that you are dealing with metrics that are hard to pin down. For example, how frequently should

I don't think it's so much technical difficulty as much as it is that you are dealing with metrics that are hard to pin down. For example, how frequently should the UI code be called out to? I guess that would depend on the speed of the computer it is running on. If it's a very slow computer then stuff is just going to be slow whatever you do , doesn't mean it's crashed. Otherwise you'd end up with a situation where you have to say "sorry , your computer doesn't meet the minimum requirements to view this webpage". Also expectations are going to differ depending on the use case. For example if I load up a huge 1GB spreadsheet I can live with the fact that the spreadsheet program is going to be unusable for a minute or two while it loads the data. On the other hand if I click on a link and the page on the other end has a buggy flash applet or whatever then having my browser suddenly lock up is way more frustrating. I suppose part of this stems from the fact that sometimes a browser is a document reader and sometimes it's an application runtime but there's no way to tell which it should be for a particular page. You could add a header or something but that would just get horribly abused.