directly; head-on; right in front
Describes facing something squarely, without avoidance; often used with verbs like 向き合う, 受ける, ぶつかる.
See also: まとも
He spoke looking me straight in the eye.
Implies taking something seriously, dealing with it honestly, or in a fair/decent manner; often used with verbs like 取り合う, 相手にする, 考える.
The typhoon hit this area head-on.
I can't speak properly because of a cold.
He can't do his job properly.
You shouldn't take his jokes seriously.
If you think about it seriously, you'll see that plan is impossible.
ろくに is used only with negative verbs to mean 'not properly', while まともに can appear in both positive and negative sentences to mean 'properly' or 'seriously'.
Adverbial form of the na-adjective まとも (proper, straight). The kanji 正面 are rarely used; the word is now almost always written in kana.