I see that you collected the concurrence of altitudes proof with altitudes as radical axes at https://www.cut-the-knot.org/triangle/altitudes.shtml. Thank you, thank you, thank you. There are 2 little typos that mar the beauty of the proof (just kidding). First, there is this invalid HTML code

Circles C_a and C_b</p>

in the second line that causes an unwanted line (or paragraph) break. Second, in the last but one line

"the pairwise radical axis of three circles concur" should be
"the pairwise radical axes of three circles concur"

Regards, Vladimir