Strict-Transport-Security response headers can cause problems for localhost web servers because STS applies host-wide, across all ports. This causes compatibility problems for web developers testing locally as well as end-users who use software packages that commonly spin up localhost webservers for ephemeral reasons (e.g. communication of an auth token from a web login to a local software package). If one local listener sets Strict-Transport-Security on a localhost response, it will be applied to all subsequent localhost requests regardless of port. We resolve this problem by ignoring Strict-Transport-Security headers on responses from localhost URLs. The HTML Fetch algorithm has been updated to require this behavior.