content has changed.  While a filtering company could choose not to use these techniques, due to the overblocking errors they introduce, if a filtering company does not use such techniques, its filter will be ineffective at blocking access to speech that falls within its category definitions.  Thus, while it would be easy to design, for example, a filter that blocks only ten Web sites, all of which are either obscene, child pornography, or harmful to minors, and therefore completely avoids overblocking, such a filter clearly would not comply with CIPA, since it would fail to offer any meaningful protection against the hundreds of thousands of Web sites containing speech in these categories.  As detailed in our findings of fact, any filter that blocks enough speech to protect against access to visual depictions that are obscene, child pornography, and harmful to minors, will necessarily overblock substantial amounts of speech that does not fall within these categories.

This finding is supported by the government’s failure to produce evidence of any filtering technology that avoids overblocking a substantial amount of protected speech.  Where, as here, strict scrutiny applies to a content-based restriction on speech, the burden rests with the government to show that the restriction is narrowly tailored to serve a compelling government interest.  See Playboy, 529 U.S. at 816 ("When the Government restricts speech, the Government bears the burden of proving the constitutionality of its actions."); see also R.A.V. v.  City of St. Paul, 505 U.S. 377, 382 (1992) ("Content-based regulations are presumptively invalid.").  Thus, it is the government’s burden, in this case, to show the existence of a filtering technology that both blocks enough speech to qualify as a technology protection measure, for purposes of CIPA, and avoids overblocking a substantial amount of constitutionally protected speech.  Here, the government has failed to meet its burden.  Indeed, as discussed in our findings of fact, every technology protection measure used by the government’s library witnesses or analyzed by the government’s expert witnesses blocks access to a substantial amount of speech that is constitutionally protected with respect to both adults and minors.  In light of the credited testimony of Dr. Nunberg, and the inherent tradeoff between overblocking and underblocking, together with the government’s failure to offer evidence of any technology protection measure that avoids overblocking, we conclude that any technology protection measure that blocks a sufficient amount of speech to comply with CIPA’s requirement that it “protect[] against access through such computers to visual depictions that are – (I) obscene; (ii) child pornography; or (iii) harmful to minors” will necessarily block substantial amounts of speech that does not fall within these categories.  CIPA Sec. 1712 (codified at 20 U.S.C.  Sec. 9134(f)(1)(A)).  Hence, any public library’s use of a software filter required by CIPA will fail to be narrowly tailored to the government’s compelling interest in preventing the dissemination, through Internet terminals in public libraries, of visual depictions that are obscene, child pornography, or harmful to minors.