Q: safari 6.1.1 does not play youtube videos. Can someone help me please? Problem started today.

Either upgrade the OS to a more current version or try another browser which is more up-to-date.

Firefox

Opera Browser

I have an early 2008 iMac running El Capitan and have no problems. I would make sure you have at least 4 GB of RAM and more is better. The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I now have 6GB installed on a early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing 

One option is to create a new partition (~30- 50 GB), install the new OS, and ‘test drive’ it. If you like/don’t like it it, you can then remove the partition. Do a backup before you do anything. By doing this, if you don’t like it you won’t have to go though the revert process.

Check to make sure your applications are compatible.

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

Open Disk Utility, select your hard drive (step 1), then the Partition tab (step 2), and select the partition. Using the /// at the bottom move it up (step 3) until the size box decrease by about 50 GB. Select the newly created space and hit the + button (step 4). Name it something and select Mac OS Extended (Journaled) as the format (step 5). Then hit the Apply button(step 6). Download the installer from the App Store and when it starts, point it at the new partition. You might want to make a copy of the installer outside the Applications folder to avoid having to re-download it in the future. Once installed, go to System Preferences/Startup Disk, select the new partition and reboot. Test away.

Safari - Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues

Safari doesn't load a page or webpage items are missing

Safari/other browsers – Website not loading