Add padding to demo elements, mark non-working example

[jamesnw]

Description

Some examples were appearing different due to lack of space around a containing block, and the changes from safe alignment here- https://anchor-positioning.oddbird.net/position-area#no-block-space

I also noted an example that no longer works in polyfilled and non-polyfilled environments.

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[jgerigmeyer]

I also noted an example that no longer works in polyfilled and non-polyfilled environments.

Worth filing an issue for that separately?

[jamesnw]

I also noted an example that no longer works in polyfilled and non-polyfilled environments.

Worth filing an issue for that separately?

It's this issue - #279, although the demo seems even more broken than initially reported.