Add Extensions to biblio.

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extra : rebase_source : c3ed632fe02fe1a42aad66f3b0eb637764b0a324
[css-variables] Improve note about custom properties. Add DoC.

--HG--
extra : rebase_source : 62591cae5197bc2d00a0370920d766fa63eef35e
[css-variables] Add DoC.