the use of abortion as a method of family planning, explaining that:  Petitioners are challenging the facial validity of the regulations.  Thus, we are concerned only with the question whether, on their face, the regulations are both authorized by the Act and can be construed in such a manner that they can be applied to a set of individuals without infringing upon constitutionally protected rights.  Petitioners face a heavy burden in seeking to have the regulations invalidated as facially unconstitutional. . . .  The fact that the regulations might operate unconstitutionally under some conceivable set of circumstances is insufficient to render them wholly invalid.

Id. at 183 (internal quotation marks, alterations, and citation omitted).  In contrast, NEA v.  Finley, 524 U.S. 569 (1998), which also involved a facial First Amendment challenge to an exercise of Congress’s spending power, articulated a somewhat more liberal test of facial validity than Rust, explaining that “[t]o prevail, respondents must demonstrate a substantial risk that application of the provision will lead to the suppression of speech.”  Id. at 580.  Against this background, it is unclear to us whether, to succeed in facially invalidating CIPA on the grounds that it will “induce the States to engage in activities that would themselves be unconstitutional,” Dole, 483 U.S. at 210, plaintiffs must show that it is impossible for public libraries to comply with CIPA’s conditions without violating the First Amendment, or rather simply that CIPA will effectively restrict library patrons’ access to substantial amounts of constitutionally protected speech, therefore causing many libraries to violate the First Amendment.  However, we need not resolve this issue.  Rather, we may assume without deciding, for purposes of this case, that a facial challenge to CIPA requires plaintiffs to show that any public library that complies with CIPA’s conditions will necessarily violate the First Amendment and, as explained in detail below, we believe that CIPA’s constitutionality fails even under this more restrictive test of facial validity urged on us by the government.  Because of the inherent limitations in filtering technology, public libraries can never comply with CIPA without blocking access to a substantial amount of speech that is both constitutionally protected and fails to meet even the filtering companies’ own blocking criteria.  We turn first to the governing legal principles to be applied to the facts in order to determine whether the First Amendment permits a library to use the filtering technology mandated by CIPA. 3.  Level of Scrutiny Applicable to Content-based Restrictions on Internet Access in Public Libraries