Learn Russian: Читая текст, любой корректор выделяет деепричастие запятыми. - While reading the text, any proofreader sets off a verbal adverb with commas.

The phrase читая текст is a verbal adverb phrase. In Russian, a verbal adverb and its dependent words must always be isolated (set off) by commas, regardless of whether the phrase comes before or after the main verb.

Why use the imperfective verbal adverb читая instead of a perfective one?

The imperfective form читая (while reading) shows that the action happens at the exact same time as the main verb выделяет (sets off). If the reading was completed before the commas were added, you would use the perfective form прочитав (having read).

Why didn't the ending of деепричастие change even though it's the direct object?

It actually is in the accusative case because it is the direct object of the verb выделяет. However, деепричастие is a neuter noun, and for all neuter nouns, the accusative form looks exactly the same as the nominative form.

Why is the word for "commas" (запятыми) in the instrumental case?

The instrumental case frequently shows the tool or instrument used to perform an action. Here, the proofreader isolates the phrase by means of commas, so the plural nominative запятые changes to the instrumental запятыми.