Some stories are posted with such a need for proofreading that it amounts to a stylistic choice by the writer; others are almost perfect. Most, by and large, are in need of substantial combing for

-typos, some of which are amazingly recurrent ('their' for 'there' and vice-versa), some others being almost unique to a writer - I can tell some writers by their typos;
-lazy turns of writing ('could care less', 'youre' for 'you're', etc);
-worst of all, inconsistencies, changes of proper nouns, sometimes complete garbling of paragraph orders:
-some choices in typing - I loathe the fashion of de-capitalizing proper nouns and pronouns which refer to slaves. Sorry Slave Ruthie! It makes for stilted reading. I always reverse it in my personal copies.

So I almost always read onscreen with a red pencil, so to speak. Yet I won't offer my services for proofreading. (well, I did once, but it came to naught). If a story does not grab me by my baddest parts, I find no interest in removing the mud and slime from its diamonds.