Publishing on arXiv in preparation for WWW conference
Since the conference will make  the papers from the proceedings open access "The proceedings of WWW are published online (open access) and through ACM Digital Library, and it is considered one of the most impactful conferences in computer science." I don't think the arxiv preprint should be a problem. However, you should e-mail first to the WWW2017 committee to see if this is against the rules.