How can I nullify css property?

Question

You have to reset each individual property back to its default value. It's not great, but it's the only way, given the information you've given us.

In your example, you would do:

.c1 {
    height: auto;
}

You should search for each property here:

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference

For example, height:

Initial value : auto

Another example, max-height:

Initial value : none

---

In 2017, there is now another way, the unset keyword:

.c1 {
    height: unset;
}

Some documentation: https://developer.mozilla.org/en-US/docs/Web/CSS/unset

The unset CSS keyword is the combination of the initial and inherit keywords. Like these two other CSS-wide keywords, it can be applied to any CSS property, including the CSS shorthand all. This keyword resets the property to its inherited value if it inherits from its parent or to its initial value if not. In other words, it behaves like the inherit keyword in the first case and like the initial keyword in the second case.

Browser support is good: http://caniuse.com/css-unset-value

.c1 {
    height: unset;
}

The unset value added in CSS3 also solves this problem and it's even more universal method than auto or initial because it sets to every CSS property its default value and additionally its default behawior relative to its parent.

Note that initial value breaks aforementioned behavior.

From MDN:

Like these two other CSS-wide keywords, it can be applied to any CSS property, including the CSS shorthand all. This keyword resets the property to its inherited value if it inherits from its parent or to its initial value if not.

like say a class .c1 has height:40px; how do I get rid of this height property?

Sadly, you can't. CSS doesn't have a "default" placeholder.

In that case, you would reset the property using

 height: auto;

as @Ben correctly points out, in some cases, inherit is the correct way to go, for example when resetting the text colour of an a element (that property is inherited from the parent element):

a { color: inherit }

An initial keyword is being added in CSS3 to allow authors to explicitly specify this initial value.