"Absolute" or "absolutely"?

Absolute is an adjective and is thus used to modify a noun or pronoun. Absolutely is an adverb and is used to modify a verb, adjective or other adverb.

Since you are modifying positioning, which is a verb, absolutely is correct. If you were talking about the position, it would be absolute.

I would phrase the answer this way:

You added position: relative, but don't seem to need it, either -- unless you intend to apply absolute position to something inside the div at a later point.

CSS does not necessarily follow the conventions of human language; it is not entirely consistent within itself. Different writers may interpret its properties and rules into different parts of speech when translating it into prose, and the modifiers those properties and rulse accept will differ accordingly. Thus,

• If the property is treated as a noun, it naturally takes adjective modifiers: the positioning [noun] is absolute [adjective].
• You could consider the property and rule together as a compound noun: we use absolute positioning [noun]
• If the property is treated as a verb form, it takes an adverbial modifier: the block is positioned [verb] absolutely [adverb].

Similarly,

• the elements use an inline [adj.] display [n.] vs. we display [v.] the elements inline[adv.]*
• we apply a left [adj.] float [n.] to the block vs. we floated [v.] the block left [adv.]