In Chrome, the static snapshots of web pages are captured and saved in a modified MHTML format (RFC 2557), that is a web page archive format used to combine resources together with HTML into a single file. A set of modifications, as summarized in this doc, are made to the existing MHTML generator and loader in order to improve security and privacy and ensure the snapshot mimics as close as possible to the original page.
There are ongoing efforts to try to add better support for creating packages of files for use on the web, as drafted in the Packaging on the Web spec. But it is a long way to get all things sorted out and agreed upon. So Chrome builds its own support upon the existing MHTML spec with improvement described in this document.