The original concept of a mythical healer was one that took wounds by touch from others, and had abilities to heal themselves using various techniques.
So in essence, they would heal the same way a rogue would dps ... exceptional healing on a single target via melee hits and mild close AoE healing.
Very focused on single target healing (e.g. tank healing), with abilities to heal damage on themselves as required. They'd probably have a health pool as good as a tank, and avoidance etc like any other melee.
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Well my Arcane mage has more in play buttons now than any time in almost the last 10 years.