The images representing the 4 different types of elements in the build tree don't have associated alt text. This makes it slower for a blind user to understand the hierarchy and navigate to the required build.

The images in question are spoken by my screenreader as, eg, elbow dash end dash minus dot gif.
Please could I suggest the following:
elbow-plus: expand not last
elbow-end-plus: expand last
elbow-minus: collapse not last
elbow-end-minus: collapse last