Steam Runtime 'Heavy' is a 64-bit only runtime based on Debian jessie.

This is a transitional runtime used for the 'steamwebhelper' component of the Steam Client.

'steamwebhelper' and 'libcef.so' included in the Steam Client are compiled in this environment,
and libraries from this runtime are also distributed in the Steam Client to provide a
suitable runtime environment for it.

You can obtain the source code and binaries from these packages from the directories below.