There are many useful console commands to use in hud editing.
An extremely common command used in hud development, this allows you to reload the hud while ingame. It has several limitations however, since it does not reload the clientscheme, and doesn't reload most menu elements. It is recommended to set vgui_cache_res_files to 0 when trying to reload hud files.
Changing the value of mat_antialias achieves a result similar to hud_reloadscheme, except it also reloads the majority of menu elements (but still doesn't reload the clientscheme). It also reloads all materials, and predictably takes longer, as well as changes overall visual quality of TF2. This is mostly useful when editing elements such as the main menu, which don't reload when you use hud_reloadscheme.
Note that by default mastercomfig blocks the mat_antialias command. You can prevent that, by adding alias block_antialias to your user/autoexec.cfg file.
vgui_drawtree is a command that allows you to see the name and some details on every hud element currently rendered. Recommended to change vgui_drawtree_draw_selected to 1 (or tick the box for Highlight Selected in the VGUI Hierarchy window), to highlight your selected on screen hud item when traversing through the tree with hud elements.
Shows the colors, borders, and fonts that a scheme file defines. It can either take no argument (which will show the clientscheme) or an argument for the scheme name.
As a list of valid schemes:
showschemevisualizer sourcescheme
showschemevisualizer sourceschemebase
showschemevisualizer chatscheme
showschemevisualizer videopanelscheme
Emits a caption, giving the caption name as an argument. Both useful for testing captions, and developing unique hud features (such as custom scripting menus) with captions.
Damages or heals the player. Positive numbers damage the player by that much, negative numbers heal the player by that much. Useful for testing health values and animations.
Determines whether to use variables that end in _minmode if present, or the default otherwise.