You can see DecompressionStream in action in unarchiver, this is the relevant code snippet (run it in Chrome 79+ with the chrome://flags/#native-file-system-api and the chrome://flags/#enable-experimental-web-platform-features flags set).

On Thu, Nov 28, 2019 at 6:07 AM Adam Rice <ricea@chromium.org> wrote:

I am trying to gauge feedback on compression streams with a view to shipping them in Chromium.

Very briefly, they are a way to do gzip and gunzip in the browser. Less briefly, the explainer https://github.com/WICG/compression/blob/master/explainer.md goes into some detail of the how and why. The specification https://wicg.github.io/compression/ gives verbose detail of how. You may also find the W3C TAG review https://github.com/w3ctag/design-reviews/issues/410 interesting.