With Safari open, press Command , (comma) to open Safari preferences then select the Advanced tab.
Select: None Selected from the Style sheet pop up menu.
If that was alreadly selected, if your see Develop in the Safari menu bar between Bookmarks and Window, click Develop. If you see any ✔ marks, click each one to deselect then quit and relaunch Safari to test.
Thanks Carolyn, but I checked the preferences and None Selected is already set in the Style sheet options.
There is nothing ticked in the Develop menu.
I have relaunched Safari, restbooted the system and even reinstalled Safari but still have the same issue.!!
I'm sure I must be missing something but have no idea what else to try.
Maybe an incompatible extension or third party plugin.
From the Safari menu bar click Safari > Preferences then select the Extensions tab. Turn that OFF, quit and relaunch Safari to test. If that helped, turn one extension on then quit and relaunch Safari to test until you find the incompatible extension then click uninstall.
If it's not an extensions issue, try troubleshooting third party plug-ins.
Back to Safari > Preferences. This time select the Security tab. Deselect: Allow all other plug-ins. Quit and relaunch Safari to test.
If that made a difference, instructions for troubleshooting plugins here.