Easy to learn
Easy to debug and test
Developers know this stage well. Debugging and testing can be a challenge. In fact, it is always a problem. Now, imagine compounding the issue using a difficult to debug and test program.
Browser interpretation varies. A program will be interpreted differently by varying browsers. What does this mean? It translates to additional work in the development stage. You have to test the program on several browsers before releasing it. That is the only sure way to establish which browsers are compatible and which ones are not.