658 ASHCROFT v. AMERICAN CIVIL LIBERTIES UNION Syllabus not harmful to minors and fail to catch some that are, the Government has not satisfied its burden to introduce specific evidence proving that filters are less effective. The argument that filtering software is not an available alternative because Congress may not require its use carries little weight, since Congress may act to encourage such use by giving strong incentives to schools and libraries, United States v. American Library Assn., Inc., 539 U. S. 194, and by promoting the development of filters by industry and their use by parents. The closest precedent is United States v. Playboy Entertainment Group, Inc., 529 U. S. 803, which, like this case, involved a content-based restriction designed to protect minors from viewing harmful materials. The Court there concluded that, absent a showing that a less restrictive technological alternative already available to parents would not be as effective as a blanket speech restriction, the more restrictive option preferred by Congress could not survive strict scrutiny. Id., at 826. The reasoning of Playboy Entertainment Group, and the holdings and force of this Court's precedents, compel the Court to affirm the preliminary injunction here. To do otherwise would be to do less than the First Amendment commands. Id., at 830. Pp. 664-670. (b) Important practical reasons also support letting the injunction stand pending a full trial on the merits. First, the potential harms from reversal outweigh those of leaving the injunction in place by mistake. Extraordinary harm and a serious chill upon protected speech may result where, as here, a prosecution is a likely possibility but only an affirmative defense is available, so that speakers may self-censor rather than risk the perils of trial. Cf. Playboy Entertainment Group, supra, at 817. The harm done from letting the injunction stand pending a trial on the merits, in contrast, will not be extensive. Second, there are substantial factual disputes remaining in the case, including a serious gap in the evidence as to the filtering software's effectiveness. By allowing the preliminary injunction to stand and remanding for trial, the Court requires the Government to shoulder its full constitutional burden of proof respecting the less restrictive alternative argument, rather than excuse it from doing so. Third, the factual record does not reflect current technological reality-a serious flaw in any case involving the Internet, which evolves at a rapid pace. It is reasonable to assume that technological developments important to the First Amendment analysis have occurred in the five years since the District Court made its factfindings. By affirming the preliminary injunction and remanding for trial, the Court allows the parties to update and supplement the factual record to reflect current technology. Remand will also permit the District Court to take account of a changed legal landscape: Since that court made its factfindings, Congress has passed at least two