nothings explicitly required (besides making sequential images that can be uploaded to the web) to make a webcomic. theres no rules (besides the definition of comics and their presence on the internet) to making webcomics.
scriptwriting is useful to many people, its really useful if the writer and artist are different people. its useful to be able to detail all your thoughts verbally, but also cant fully communicate your comic vision. its a potentially useful step of many - and every comic creator does scripting differently.
personally, i used to translate prose to an adapted script to thumbnails to pages. adapting prose to comics proved difficult in a lot of ways, and i think both my comics and prose suffered for the relationship a bit. nowadays i jot out the happenings, create a page by page of events - page 1 steve goes to the fridge, page 2 steve looks for milk, page 3 aliens attack. from there i script as i draw, and the script is more of a verbal aid to the visual aid - a jotting down of visuals, order, key details, and dialogue.
im sure it would be possible to make a comic with zero scripting at all, just thumbnails and sketches and more sketches - for a short i made / am making, which starts with essentially a montage, i developed a list of potential visuals and then went straight into thumbnailing and arranging. but it was a nonverbal scene - the next scene has a script attached.
this all said - i dont think its a good idea to make a webcomic without planning of some kind. that doesnt need to be scripting, it can be several layers of thumbnailing and editing, it can be 100% visual with no words. it can be various notes and jottings that only make sense to you. however you work - but just sitting down with your page file and going for it is gonna be a painful way to work.
speaking of things without scripts though, this is a really interesting video on how mad max: fury road was way more storyboard based than script based, and how that aided the visual language of the film. the storyboards, it looks like, can arguably be considered a comic (as all storyboards can)