Online contracts are enforced where the user had reasonable notice of the terms and manifested assent. How the interface is designed decides both.

Clickwrap. The user affirmatively clicks to accept, with the terms available by a conspicuous link adjacent to the button. Routinely enforced.

Browsewrap. Terms available by a link somewhere on the page, with assent inferred from use. Routinely unenforced absent actual knowledge.

Sign-in wrap. A statement adjacent to the action button that proceeding constitutes acceptance, with a hyperlink. Enforceability turns on conspicuousness — font size, contrast, placement, and whether the hyperlink is visually identifiable as a link.

Evidence of assent. Retain the interface as presented on the date of the user’s action, the version of the terms then in force, and the account record of the action. Screenshots of the current site do not prove what a user saw three years ago.

Modification. Terms reserving a right to amend by posting are frequently unenforceable as illusory unless notice is given and continued use follows. Emailed notice with a stated effective date is the defensible approach.

Arbitration and class waivers live or die on this analysis, which is why interface design is a legal question as much as a product one.