I've also experienced a somewhat similar issue with creating LR Web Galleries from raw images referenced in Collections - I'm finding they reproduce somewhat warmer and darker than the images warrant, but it isn't the kind of train-smash you are describing. Now, as a workaround I export these images to Photoshop and use my own set of conversions for turning these images into web material (basically re-sizing, convert to profile sRGB with BPC, then convert to 8-bit mode and save as JPEG) , then re-import them to LR as JPEGs and then let LR create a web gallery from those. I'm finding that version comes out better. Try this with your problem gallery and let us know whether it changes the outcome. The fact that different processing approaches lead to different outcomes (in my case) suggests that it may not be a browser colour-management issue, and one would assume Adobe knows much of the world views these galleries through Internet Exploder so they program the algorithm to deal with that, but I still wouldn't rule out browser colour management as part of the problem.