What happens if we only write opening HTML tag?

Answer 1

By only adding opening tags, it would technically make every tag after it a child of that element. For example, if, in your body tag, you have multiple different p tags, all with no closing tag, all of the p tags will be the child of the first p tag which is a child of the body tag etc.

To sum it all up, nothing really works the way it is supposed to.

Answer 2

it would be FUBAR