Web dev here. It enforces the original markup and code from a server to be the markup and code that the browser interprets and executes, preventing any post-loading modifications.

That sounds a bit dry, but the implications are huge. It means:

• ad blockers won't work (the main reason for Google's ploy)
• many, if not most, other browser extensions won't work (eg.: accessibility, theming, anti-malware)
• people are going to start running into a lot of scam ads that ad blockers would otherwise prevent
• malicious websites will be able to operate with impunity since you cannot run security extensions to prevent them
• web developers are going to be crippled for lack of debugging ability

These are just a few things off the top of my head. There are endless and very dangerous implications to WEI. This is very, very bad for the web and antithesis of how it's supposed to be.

TBL is probably experiencing a sudden disturbance in the force.