iframe with HTML (not XHTML) fail to load

[danielweck]

See:
readium/readium-js-viewer#723 (comment)

Instead of loading, a popup download is displayed. This looks like an overzealous / badly-configured security rule in the Electron webview setup. To be verified.

[danielweck]

I cannot see anything suspicious in https://github.com/readium/r2-navigator-js/blob/develop/src/electron/main/sessions.ts