In 2011, Matt Cutts, director of Google’s webspam department, was asked if spelling and grammar are essential in evaluating the content and quality. The interviewer begins with a statement saying that since he last checked spelling and grammar, they were not ranking signals directly.
Cutts states that through various tests, Google has determined that websites with more punctuation and spelling are more reliable.
Cutts concludes that content should offer users a positive experience, regardless of its effect on search engine rankings.