I've exported and uploaded a video to YouTube that was shot on a white background, and has been masked and color corrected to appear seamless with a white solid in the BG.
What should appear as 100% white doesn't when displayed on certain monitors. I can't see it on my iMac, but it appears in a thumbnail when posted on Facebook and displays in the video on certain PCs (pictured here):
When I import the video in Photoshop or use an exported still frame from Premiere the RGB values all display at 255 across the mask, over the edges and onto the background solid.
The foreground (masked video) was shot with a Nikon D800 and the physical white background was boosted to 100% using Luma Curve. The electronically generated background is a white solid generated in Adobe Premiere Pro CC 2016. Output format was ProRes 422, compressed to .H264 for YouTube.