Dynamic Logging window for a webpage

I'm building a website on which users can draw objects.

I want to build a window for logging whatever mistakes that users make

For example, the drawing rule is only circle is allowed inside a triangle. However, one user come and draw a square inside a triangle. The system still allows and perform it as the user wants. But there will be a warning message such as "only circle allowed inside a triangle" appeared in the logging window (located at the bottom of the page). Whenever the user fix the mistake, the warning in the window will be deleted.(Due to this function, I think it should be called dynamic logging window)

I'm using JavaScript with ExpressJS and dojoJS.

Can anyone tell me how to build such a window??? I'm quite new to web developing

Any help would be appreciated.