As a non-native English speaker I have to disagree here. Was using the web long before I could get those things right. And I can assure you many Swedes would fail to get English particles and prepositions right. And they are still able to use most English websites without much if any problems.

Getting particles (eg. "in" in "sign in") and prepositions right is one of the hardest parts of learning a new language. (Word gender for languages with that feature is another hard part.) You can understand almost everything of a website just from understanding verbs and nouns.