This is perfect opportunity to learn the basics of CSS specificity .
In short, the key thing is that it is the complexity of the selector and the order in the stylesheet which effectively determine what element is targeted.
I would bet , in your original Q, that .main p is after .content p in your stylesheet. Since both rules have the same specificity the second merely overrides the first.
This may not apply to your current case, BUT…
If you are able to alter (simply your source code) you could do:
<div class="main">
<p>text text</p>
<p>text text text</p>
</div>
.main p /* .main p rules*/
.main p + p /* .content p rules; you could also do .main p ~ p */
You save an extra div tag, and class name. do note, as long as we are on the subject of specificity that .main p + p > .main p .