"Harassment based on race or color can include unwelcome verbal, written, or physical conduct directed at the characteristics of a person's race or color such as nicknames emphasizing stereotypes, racial slurs, comments on manner of speaking, and negative references to racial customs."
In this context, shoudn't "manner" be preceded by "a", as several dictionaries show "manner" (in the sense of a way of behaving) is countable?
What are the grammar rules on when a countable noun becomes uncountable?
With regard to your question about when countable becomes uncountable, this can occur when the meaning of the noun changes from being considered a specific, countable entity (Hairs found at the crime scene were traced to the suspect.) to an abstract or non-specific/non-countable entity (Hair can be styled in many different ways.) Some nouns cannot be considered both count and non-count. I understand that this is a difficult area for English learners. Some nouns that can or cannot be considered both, simply must be memorized. Check the web for more detailed info and examples.