improve the http aio example by:
- adding a background color to the messages section so that it
can be distinguished from the others
- avoid using `hr` elements to divide the various sections, use
css instead (so that the divisions can also be omitted when
the sections are not being shown)
- fix the erroneous presence of an input text element inside a button
(see: https://github.com/angular/angular/pull/44557#discussion_r787239658)
PR Close#44986
