I think that exclusively the online element wouldn’t determine my approach to any of these things, so much as the context – i.e. the site, or publication, etc. – for which I was writing, and the guidelines they work within. Which are often, though not necessarily, different from those in print. Given free-reign, say a neutral (ha!) platform to write however I wanted – I would probably be inclined to change very little, though might assume it would be appropriate to cut down on length and breadth, as these are the things I struggle with – re: attention – when reading online.
Length is the main structural aspect that I feel more relaxed about online – there is no graphic design grid or page number that puts parameters around how much I can write. That said, I don’t believe in using all that space to ramble on indefinitely – I always try to err on the side of concision rather than digression where possible. As for breadth of research and references, I don’t see why one would pay any less attention to that online that one would in print. Regarding images, there’s scope to include more (as there is greater flexibility with word lengths) and images of different quality – the issue with print is one of resolution, which isn’t so much of a concern (and is even a feature of certain types of image) online.
I think that the trope of online writing – essentially a form of public diaristic commentary, with added hyperlinks – is there, and that has come to be expected by writer and reader: shorter, less research, referencing only online sources. I consciously try to avoid this, and keep a tone and atmosphere that would sit in either. It my be delusional on my part.
That said, of several online articles, one was commissioned as a ‘Postcard From…’ on a blog, in which writers report from a location travelled to; the editor encouraged including how you got somewhere, and images taken while on the road, things I would never otherwise incorporate into my writing. So there has been cases where the publishing host has called on parts of the blog trope.
In order: generally shorter; more breadth, less depth; more images; more references to online sources (via hyperlinks, embeds).
The length of the piece really depends on the outlet. Online publishing often comes with an accelerated publishing schedule. Everything has become more compressed, a site has to change a lot to be seen as active, which creates an insatiable demand for content. This leads logically (and very unfortunately) to smaller research and editing time. The importance of things like images and headlines, i.e. the whole “package” around the piece itself, is exacerbated since these are often what will trigger the click.