no green background

This is easy but it requires us to move into new territory.
So, once again, I'd like to work with the simplest possible file.
So I need you to create a play3.html file with just this content:

<img src='cards/back.png'>
<img src='cards/back.png'>

To keep things simple earlier, I neglected to mention that these lines should be enclosed in body tags:

<body>
  <img src='cards/back.png'>
  <img src='cards/back.png'>
</body>

And I can now introduce you to a new attribute called style.

Try this:

<body style='background-color:green;'>
  <img src='cards/back.png'>
  <img src='cards/back.png'>
</body>

A reminder of what I said earlier, "In the olden days, browsers could do little more than display pages with links to other pages."
A new language had to be invented to allow us to make web pages more attractive.
This language is called CSS and background-color:green; is an example.
background-color is called a CSS property.