What is the meaning and usage of the word "very" in the following sentence?

Solution 1:

Here, the word "very" is used to put additional emphasis on the word it modifies: "first". It's a shorthand for saying

XSLT is a language that, according to the specification — in fact, not just any place in the specification, but the first sentence of it —, is primarily designed for transforming one XML document into another.

Or:

XSLT is a language that is primarily designed for transforming one XML document into another — that's what the specification at http://www.w3.org/TR/xslt20/ says right in its first sentence.

Solution 2:

In this case, "very" is used to give emphasis to "the first sentence."