Alter style in JavaScript without affecting what gets rendered in the browser

I extract several blocks that are spread across the page, draw them onto a canvas, via Canvas​Rendering​Context2D.draw​Image(), and then I export the canvas to PDF/JPG.

Now, while drawing them, I also want to tidy their looks and/or remove/display parts of some blocks, because the styling no longer makes sense from a static point of view.

I can append a class on body, style blocks differently while body has that class, build the canvas, export it to a content type of choice and remove body class, but this makes the page look quirky for 1 - 2 seconds. (I could just cover the page with a loader, I guess)

I can clone the block, style it differently or apply whatever styling (inline or via classes) and draw an image from that, but this is not optimal.

Any idea on how to do this properly? I'm inclined to go for something like emulation. Have the whole "export styling" under the native @print and emulate that before drawing the blocks onto the canvas, without affecting what gets rendered in the browser, but I'm not sure if this is possible.

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