It looks working fine.
However when Debug.log is called right after the Elm page rendering (around init function, for instance), that first debug print is not handled by the extension (shown in default style).
After the page load settled, subsequent debug prints are correctly handled by the extension and formatted. Is it intrinsic limitation of browser extension? By that I mean, browser extensions are lazily loaded after the page load so they cannot intervene events that happened before they are loaded?