As we now know its due to the browser caching the site.
In short: It saves the CSS/HTML pages so you don't have to get it again.
I recommend the same link as Patsy said: READ ABOUT BROWSER CACHE
So to get past this problem we can tell the bowser to get the HTML/CSS again using:
Shift + F5 or CTRL + F5.
What it says on that page:
A cache is a repository for stored data that is used to expedite the
process of retrieving data. There are several types of caches,
including webpage caches, hardware caches, software caches and memory
caches. Caches are used to speed up a process so that data does not
have to be recomputed or fetched from its original location and,
therefore, saves time. The greater the requests that can be served
from the cache, the quicker system performance becomes. A cache hit is
when a program checks the cache for previously stored information and
finds it. A cache miss is when the data is not found within the cache
and the program must look to a larger, slower memory source. Computers
can have several caches: the L1 cache is built into the computer
processor’s chip, the L2 cache is built onto the motherboard and the
L3 cache is just the L2 cache on a computer that has two caches built
into the microprocessor.
Internet browsers use caching to store HTML
web pages by storing a copy of visited pages and then using that copy
to render when you re-visit that page. If the date on the page is the
same date as the previously stored copy, then the computer uses the
one on your hard drive rather than re-downloading it from the
internet.