SQL "Standard" guide?
The idea that an application should be written with just standard SQL comes from application developers who think that changing a DBMS is a worthy design goal. I -- and many other DBAs with me -- don't agree.

While it holds true for basic features, your application is going to perform best if it uses the features that your particular DBMS provides. This almost always means using DBMS-specific versions of SQL and DBMS-specific features.

Your employer/client has paid good money for a license for their DBMS, and often gone through a selection process to do so. Not using the DBMS's features is a waste of money.

I like to say it's like buying a Ferrari and always sticking to the speed limit...

On the other hand, if you're just learning SQL, and T-SQL just happens to be the dialect you have at your disposal, then, yes, try to learn standard SQL as your knowledge will be more broadly applicable. But don't discount learning your particular DBMS's features if you are in a commercial (or non-academic) environment.