Question

I found both of them works in my test:

    .on("mouseover",
        function() {
            d3.select(this)
                .select("text")
                .style("fill","red");
        })

or

    .on("mouseover",
        function() {
            d3.select(this)
                .select("text")
                .attr("fill","red");
        })

Answer 1

If you look at the HTML you get, you’ll see something like:

<text style="fill: red">...

and

<text fill="red">...

..which are both legal in SVG, but using attr when you need style could trip you up if you use it for something else.