One advantage is I have 5 year old) keynote presentations that keynote refuses to open (tells me file format is too old), and 3 year old PowerPoint slides the new version messes up the formatting of. I'm slightly more confident my HTML will continue to render right, and at least open!
Portability. If you have to send a presentation around, embedding fonts in PowerPoint only works if the viewer is on a Windows machine, and keynote is not an option if your viewer is on a PC.
One is that you can't have speaker notes integrated in the presentation the same way you can with PP or reveal.js.
The other one is that it doesn't scale. You export the slides in one display ratio and it stays like that. Most web presentation formats will adjust to whatever your display is doing.
I often have complex animations and embedded video that will not survive the PDF export, and am sending presentations to people who can't just be asked to install libreoffice.
The coolest thing ever is when a talk at a conf has the URL for the HTML slide deck, so if you miss something on a prev slide you can go back and check, & you don't need to figure out where/if the slides were published when you want to revisit after the talk.
in addition to what others have said: teamwork. Coordinating a group working on Powerpoint slides is quite annoying, text-based formats are a lot easier to manage and merge, especially for technical folk familiar with git & co. Google Slides is the only GUI variant I know that deals with this by syncing changes immediately, and it has other weaknesses.
I'd say so, yes. Mostly just because HTML is ubiquitous, it means your presentation continues to live on after your talk by uploading it to your site. Plus your slides can be responsive, accessible, searchable, linkable (if you build them correctly of course).
I think you got it backwards... right now non-tech-savvy people focus on the content with their tooling (PowerPoint, Keynote, Google Slides). Your tooling introduces a new layer of complexity: now they have to learn HTML and css to do the _exact same thing_. What you built looks great, but you have to reconsider who you're marketing it towards. I can't imagine anybody but a programmer using this.
If your goal is to build a platform for presentations like medium is for articles, isn't that what speakerdeck.com already is? If you want to build a better speakerdeck, just say that then? "A better way to create and share slides online"
This is the first release of WebSlides. I don't want to build a better Speakerdeck. WebSlides is about telling/sharing engaging stories (css, indexable content...).