WordPress 4.7.1 Breaking on Embedded Divs and Javascript

After updating multiple Websites to 4.7.1 we have found many pages failing to render. So far I have narrowed the problem to embedded code, either DIVs or Javascript.

For example, embedding an Amazon Native Advertising widget at the bottom of a page causes WordPress to omit all but part of the HEAD section (the page becomes blank). Removing the Javascript code snippet at the bottom of the page fixes the problem.

The same code snippet can be loaded into a sidebar widget and it runs perfectly fine. I suspect it works well in other widget areas, too.

Another example concerns a page that relies on custom/bespoke embedded styling (embedded DIVs). I can get the content down to the first DIV to render. As soon as a second DIV is encountered WordPress chokes and only publishes a few lines of text in the HEAD (the page is rendered blank). The code rendered fine in WordPress 4.7.

The problem occurs in multiple themes and browsers (changing theme does not help, changing browser does not help).

I have not, as I write this, restored the older version of WordPress. That will take some effort as I am not sure what will happen with the plugins. Will post more info when I can.