The same is true for PDF files, they support forms and can even POST to a server. Would you consider PDF a format designed to build apps instead of one designed to publish documents ?
HTML is great for these kinds of 'apps', like this website. This is where HTML is fulfilling its original purpose. Reading text, filling out a few forms, that's exactly what it is for. I'm not arguing against interactive websites such as e.g. Reddit or Slashdot. What I'm arguing about is actual full-fledged apps built in HTML. Photoshop should not be a web app, nor should Visual Studio.
With classic html, you can do similar things as with PDF but it was used to write anything from IRC clients, school exams, games and a lot more. Like PDF, it contained support for scripts to allow applications not mere documents to be implemented but unlike PDF it was especially made for the web.
People did implement full fledged apps in plain html powered by powerful server side languages like php or jsp. The trend of doing a lot of heavy lifting in clients is fairly modern after clients were able to handle such workloads and internet speeds were not a deterrent. For clienr capabilities of the time, html was very much meant to implement apps in the browser with all the heavi lifiting done on the server side because pentium 1 with 500mb ram,dialup and windows 98 can't do modern types of apps even with flash well.
This is false. Various inputs and forms were in html since its early days, these are not required to simply publish and view a document but to interact with text and graphical content.
It did lack the ability to do "rich" content/apps and flash tried to fill in that gap, html5 came a bit late but I hope it is forward compatible. Would an html5 app from 2015 run fine in 2035?