Web publishers often refer to articles, videos, photos, etc. as "content" or "online content", but I feel as if this term is hackneyed jargon and also fairly vague. What alternatives are there?
The traditional and formerly more usual word is 'material'. OED, B. n. 1.:
c. Text or images in printed or electronic form....
"Content" is not hackneyed jargon; it is an HTML attribute to describe the contents of a web page. You are right in that it is vague, as it is meant to be one word that describes multiple "stuff" in the page - pictures, videos, text, and the like. If you wanted to be specific you would have to call them out as "News", "Pictures", "videos" etc.
A page could have content (sg.) encompassing various text or graphic types already in print media, as the first of the following examples shows, long before there was an internet:
Among additions to the feature page content will be a music column, short stories and at frequent intervals, crossword puzzles. — The Daily Bruin (UCLA), 17 Jan. 1936.
Editorial page content was defined as written content, cartoons, illustrations, or graphs appearing on pages clearly identified and set aside as commentary or opinion. — Michael W. Drager, Organizational Structure and Organizational Culture Influences on Editorial Page Content, 1997.
Digital media continues this usage in the same sense, always singular, as a hypernym subsuming a larger number of content types than print, such as audio and video.
In other contexts, things contained in something else are aggregated as contents, always plural — of a closet, barrel, or your kitchen drawer overflowing with odds and ends. Contents does not function as a hypernym: A broken screwdriver in your kitchen drawer is not a type/kind of contents, but a video or photo on a webpage is a type/kind of content.
It puzzles me why you would seek an alternative.