What is the Spanish dialect that now standard?

There isn't a "Spanish dialect" that is considered "standard." Instead, the standard form of Spanish is called Castilian.

It's important to note that:

* Castilian is the official language of Spain, and its pronunciation and grammar are the basis for the standard.

* Standard Spanish is a formal, codified version of the language used in education, media, and official communication.

* While Castilian is considered the standard, there are many regional varieties and dialects within Spain and throughout the Spanish-speaking world, each with its own unique characteristics.

So, while Castilian forms the basis of standard Spanish, it's crucial to understand that language is dynamic and evolves across regions.