I want to display SCADA plant in android using HTML5. The text on the images will be changing continuously. I tried using label over image and process the text using javascript. But is there any specific way for SCADA plants?
There are a lot of things to consider here.
You will most likely have to develop your own web application that interfaces with the SCADA Server (using OPC or another supported protocol) to obtain and poll the data. From there you would need to develop an API so that your mobile application (client) can talk to the web application and display the data.
Once your clients can poll the data, yes, you can update labels of images periodically using javascript. Make sure, though, that you poll only a range of tags/points/variables (such as the ones on display), rather than getting your android app to request the whole tag list every few seconds (could be huge data expenses there).
Is it an issue if unauthorised access is obtained? Any sensitive data or secure control commands? Exposing your SCADA server to the internet isn't the best idea. A basic thought though, with the solution above, you could have your web application server connected to the internet and connected locally. If you had the security policies and had a good analysis of your security, it could be possible (and potentially safe - for your purpose), for the two servers to talk.