The syntax for TML is similar to HTML, except it uses Toga attributes and tag names.
Any Toga widget is supported in TML, as long as it is imported at the top of the file with correct TML import syntax.
For instance, if the Toga code is toga.Label("Hello", style=Pack(color=#ff0000)) then the TML code is <Label "Hello" style=Pack(color=#ff0000)/>
It is very simple and very intutive if you already know Toga.
The Toga documentation can be found here to see specifics of the framework: Toga API Reference

Prefabs

Prefabs are a part of TML which isn't part of base Toga. They allow the definition of preset Toga widgets with custom tags, which may be reused later in the TML code. - All prefabs must be defined in the <Prefabs></Prefabs>. - The structure for a prefab is <CustomTagName "Argument if necessary" type="the normal Toga tag name e.g. Label" attribute1="this" attribute2=(that)/> - The prefab can then be called with <CustomTagName name="name_of_this_instance"/> - Any instance of a prefab you use in the Body must have a name="" attribute to distinguish it from other uses of that prefab. - Instances of prefabs can still be modified from their original, and attributes and arguments from the instance will be prioritised over the originals.
Example: <CustomTagName name="name_of_this_instance2" attribute1="I've changed my mind"/>