Thank you guys, helpful answers which made me read some more, and finally I read this article and removed all dependency on JavaScript, and used the nth-last-of-type pseudo-selector to apply the special background to the last li items in a list (ignoring the ul that comes after the last li).
The final code is here. I am going to accept my own answer, unless someone points out some problem with it. (I don't think compatibility with older browsers matters to me at this stage.)
Thanks to @Aram for the answer. @OneTrickPony, your answer went over the head of this noob :) I am sure it does the right thing, but it's a bit too complicated for me.
<style type="text/css">
/* ul[class=tree] and every ul under it loses all alignment, and bullet
* style.
*/
ul.tree, ul.tree ul {
list-style-type: none;
margin:0;
padding:0;
}
/* Every ul under ul[class=tree] gets an indent of 1em, and a background
* image (vertical line) applied to all nodes under it (repeat-y)
*/
ul.tree ul {
padding-left: 1em;
background: url(vline.png) repeat-y;
}
/* ... except the last ul child in every ul; so no vertical lines for
* the children of the last ul
*/
ul.tree ul:last-child {
background: none;
}
/* Every li under ul[class=tree]
* - gets styling to make it bold and blue, and indented.
* - gets a background image (tilted T), to denote that its a node
* - sets height to match the height of background image
*/
ul.tree li {
margin:0;
padding: 0 1.2em;
background: url(node.png) no-repeat;
line-height: 20px;
color: #369;
font-weight: bold;
}
/* The last li gets a different background image to denote it as the
* end of branch
*/
ul.tree li:nth-last-of-type(1) {
background: url(lastnode.png) no-repeat;
}