Either:

1. (Better) Include transpilation for older JavaScript compatibility during buildtime of static assets.
2. (Good, and best if combined with 1) Set up testing with IE.
3. (Easy) Remove uses of template literals where they have been introduced (see below) and include rules for disallowing them.

Current occurrences of template literals (upon superficial skimming):

1. karma/client/updater.js

   Line 32 in e246461

   titleElement.textContent = `Karma v ${VERSION} - ${connectionText}; test: ${testText}; ${pingText}`

2. karma/client/updater.js

   Line 56 in e246461

   updateConnectionStatus(`reconnecting in ${sec} seconds`)

3. karma/client/updater.js

   Line 74 in e246461

   updatePingStatus(`ping ${latency}ms`)

4. karma/client/karma.js

   Line 194 in e246461

   self.updater.updateTestStatus(`karma_error ${message}`)

@johnjbarton awaiting your feedback on this...