Wikipedia
.har
The HTTP Archive format or HAR, is a JSON-formatted archive file format for logging of a web browser's interaction with a site. The common extension for these files is .har.
The specification for the HTTP Archive (HAR) format defines an archival format for HTTP transactions that can be used by a web browser to export detailed performance data about web pages it loads. The specification for this format is produced by the Web Performance Working Group of the World Wide Web Consortium (W3C). The specification is in draft form and is a work in progress.