Is it possible to use use HTML tags in the Node's text of TreeView?

Yes. You can use TreeViewNode.IsTextHtml property to use html tags in the Node Text.

C#

//Get the node instance

TreeViewNode node = TreeView1.FindNode("Node");

node.IsTextHtml = true;

node.Text = "<b style=''''color: blue''''>Font";

VB

''Get the node instance

Dim node As TreeViewNode = TreeView1.FindNode("Node")

node.IsTextHtml = True

node.Text = "<b style=''''color: blue''''>Font"

Please refer the sample in the below link which illustrates the above:

http://websamples.syncfusion.com/samples/Tools.Web/5.2.0.25/KB_Treeview/main.htm