CSS in blocks affects other blocks

Accidentally I found that if I type some CSS in a block, like this:

<html> <a></a>
      table, th, td {border: 2px solid red;}

then other blocks are affected, for example the CSS above change the border of tables, if in another block I use Markdown or HTML table like this:

| Column | Column |
|--      |--      |
|Foo     |Bar     |

it is rendered like this:

(the block above is the one containing the CSS).

Is this an expected behavior?

Anyway, in the CSS block I need to write something before the style, in my example I wrote <a></a> but it can be anything, otherwise this “trick” doesn’t work. Does someone know what is happening here?