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[worldofneil]

In macros, you can also combine multiple options, like:

/micon "Cure"
/ac "Cure" <t>
/ac "Cure" <tt>
/ac "Cure" <ft>

That would try casting Cure on your target, but if that isn't possible (like when you have an enemy targeted), then it would try casting it on your target's target.

You are wrong. Macros don't automatically default to self. The example given would try to cast cure on the Target then if that's not successful, it tries to cast cure on the Target of the Target, then if THAT isn't possible, then it tries to cast cure on the Focus Target. Unless you are targeting yourself or change the first /ac "Cure" <t> to /ac "Cure" <me> then it will not cast on yourself.

Sorry, but you're both wrong (I even tried the macro above just to make sure it wasn't something strange when doing as a macro!).

Cure (and Physick etc) will always cast on yourself if they cannot cast on the target in question. This has always been the case since at least 2.0 (possibly earlier than that, but I can't remember). If you have an enemy targetted or something that is valid to receive the cure (someone who just died or just raised for example), the cure will still cast, but will always land on yourself.

You can literally confirm this by finding any mob in the world, target it and just running the first line, /ac "Cure" <t>. From what you both wrote it should error, but it won't, it'll cast on yourself without an error (and in the macro it would cause the following lines to not run as you're already casting)

[TensaiSogetsu]

My mistake, thanks Worldofneil!

[Niwashi]

Cure (and Physick etc) will always cast on yourself if they cannot cast on the target in question. This has always been the case since at least 2.0 (possibly earlier than that, but I can't remember). If you have an enemy targetted or something that is valid to receive the cure (someone who just died or just raised for example), the cure will still cast, but will always land on yourself.

That's true if you have the skill itself directly on your hotbar and use that, and I'm pretty sure it's also true if you have just /ac "Cure" in a macro. In those cases, it will try casting on your target, but will cast on yourself if that's not valid. But once you place the target specifier in the macro line, like <t>, <tt>, <mo> or whatever, then that's the only target that that line of the macro will use. Target selection like this is one of the most popular and widespread uses for macros.

[Keydah]

Select party members with the party and alliance lists, and angle your camera to only tab through enemies directly in front of you. Don't go through and macro all your skills to <tt> and whatnot, it'll just make it harder on yourself because you won't be able to queue them; this will be awful in situations where you have to spam heal someone.

[worldofneil]

But once you place the target specifier in the macro line, like <t>, <tt>, <mo> or whatever, then that's the only target that that line of the macro will use.

Sorry, but no... (just try it!)

Damage spells and actions do work exactly as you described, but healing actions are different. If you try and cast a healing spell on a mob, regardless of the method, if always casts on yourself (if it's a spell that you can cast on yourself normally).

So if the first line of the macro is using <t> it'll just cast on yourself causing the others to fail.

[Anova]

healing and buffs will default onto yourself if there is no valid target, even when given with a <t>,<tt>, or <mo> target marker.

You also should be aware that when you pace 2 or more commands like that together in the same macro, that you can accidentally trigger the 2nd or 3rd line of the macro when you spam the key.
The reason being that macros interrupt other macros (they still run all the commands in your macro but not trigger actions if you are on gcd) but the normal actions without macros are queued to run after your gcd.

[Rongway]

healing and buffs will default onto yourself if there is no valid target, even when given with a <t>,<tt>, or <mo> target marker.

It's safe to use <mo> alone. It won't redirect to self if you try to use it with an invalid target or with no mouseover.