Hi there!
I think I can help with this one.
This idiomatic expression "for what is ..." does seem a little strange. The ordering of the words is a little confusing. Let's take a look at that part of the sentence to better understand its meaning and then we will take a look at some other examples.
"...the ancient name for what is today Beijing..."
All that this says is that the other name is the same place as Beijing. It had one name in the past and another name now. So you could actually rewrite this part of the sentence without this phrase and keep the same meaning:
"...the ancient name of Beijing..."
or even
"...what Beijing used to be called..."
So using this expression is a stylistic choice by the author to say something in this way. All of these expressions convey the same idea.
Here are some other examples that show the many ways this expression can be used:
1. All people should stand up for what is right.
2. The media continued to follow the politician, demanding an explanation for what is now known as the Watergate Scandal.
3. My request was ignored for what is considered by many a petty mistake.
4. For what is said to be a high class restaurant, I was surprised to find hair in my food.
I hope that these examples give you a sense of all the ways we can use this expression. As you can see, it is usually involved in re-naming something or adding more detail to something already mentioned.
Cheers
