Why safari display webpage not properly in iMac?

[wildpointer]

I bought iMac 21.5 inch last month. The spec is the following,

21.5-inch: 2.7GHz
Specifications
2.7GHz quad-core Intel Core i5
Turbo Boost up to 3.2GHz
8GB (two 4GB) memory
1TB hard drive1
Intel Iris Pro Graphics

However, safari doesn't display webpage properly. Please refer to the attached.
I didn't have any extension installed on safari.

May I know how do I set the web browser properly?
Thanks