Because both of them are at an equal specificity (only referencing class), the one at the end of the file has precedent. If you were to make .text_left be div.text_left, then it is more specific and it will override .form_container_header no matter where it is in the file.
From W3C:
6.4.3 Calculating a selector's specificity
A selector's specificity is calculated as follows:
- count 1 if the declaration is from is a 'style' attribute rather than a rule with a selector, 0 otherwise (= a) (In HTML, values of an element's "style" attribute are style sheet rules. These rules have no selectors, so a=1, b=0, c=0, and d=0.)
- count the number of ID attributes in the selector (= b)
- count the number of other attributes and pseudo-classes in the selector (= c)
- count the number of element names and pseudo-elements in the selector (= d)
The specificity is based only on the form of the selector. In particular, a selector of the form "[id=p33]" is counted as an attribute selector (a=0, b=0, c=1, d=0), even if the id attribute is defined as an "ID" in the source document's DTD.
Concatenating the four numbers a-b-c-d (in a number system with a large base) gives the specificity.
Some examples:
* {} /* a=0 b=0 c=0 d=0 -> specificity = 0,0,0,0 */
li {} /* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,1 */
li:first-line {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
ul li {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
ul ol+li {} /* a=0 b=0 c=0 d=3 -> specificity = 0,0,0,3 */
h1 + *[rel=up]{} /* a=0 b=0 c=1 d=1 -> specificity = 0,0,1,1 */
ul ol li.red {} /* a=0 b=0 c=1 d=3 -> specificity = 0,0,1,3 */
li.red.level {} /* a=0 b=0 c=2 d=1 -> specificity = 0,0,2,1 */
#x34y {} /* a=0 b=1 c=0 d=0 -> specificity = 0,1,0,0 */
style="" /* a=1 b=0 c=0 d=0 -> specificity = 1,0,0,0 */
<HEAD>
<STYLE type="text/css">
#x97z { color: red }
</STYLE>
</HEAD>
<BODY>
<P ID=x97z style="color: green">
</BODY>
In the above example, the color of the P element would be green. The declaration in the "style" attribute will override the one in the STYLE element because of cascading rule 3, since it has a higher specificity.