Web content—live pages, archived snapshots, and social media posts—now surfaces in nearly every federal case, yet counsel forfeit it constantly by neglecting the foundation. This guide follows an invented false-advertising fight between two smart-camera makers through the three authentication problems that web evidence poses: proving what a site says now, what it said before the owner rewrote it, and who actually authored a post. It walks the Rule 901 threshold, Judge Grimm's five-gate Lorraine framework, the Rule 901(b) toolkit, the 2017 self-authentication amendments in Rules 902(13) and 902(14), the Wayback Machine problem after Weinhoffer, the account-versus-author gap after Griffin, Tienda, Vayner, and Browne, and the hearsay and best-evidence hurdles that survive authentication. It closes with the emerging federal response to synthetic evidence—Proposed Rules 707 and 901(c)—a capture-to-courtroom workflow, a method-matching table, and a practitioner FAQ.