it's just an img tag with an image that is updated a few times per minute, guessing it's a screenshot of some sensor terminal at cern? :D

Probably not

Many old monitoring programs just literally generated graphs like these using input data and outputted a static image on a cron/tick. "Expensive" once, then cached... in the form of the image

I'm guessing this is that sort of tech

These are screenshots taken from SCADA screens.

Architecture briefly explained here: https://accelconf.web.cern.ch/ica05/proceedings/pdf/O3_005.p...

I also want to mention EPICS which is used by quite a few physics and telescope projects: https://epics-controls.org/epics-users/projects/

When CERN Was looking at software they found EPICS wasn't up for the task at the time: https://cds.cern.ch/record/532705/files/ma1o06.pdf