There's no need for a default inline-block container. Inline-block content is usually block content, so you just use a <div> and apply
Where's my default inline block container?
<div> is display: block by default. <span> is display: inline by default. Where's the default inline-block element? Also, I feel each major display type should have a corresponding element. <grid> for display: grid, <flex> for display: flex and so on. As for inline block - I'd vote for <IB> as its default container.
Yes, it would be nice – maybe if you suggest it to the open source community or to Opera they may be able to build it into a future version! Of course, running your pages through the validator is a pretty easy way to check (and Opera certainly makes this very easy, just right-click on the page and Validate is there on the context menu!) or, as Robert says, you could run them as XHTML.
Browsers will try to render anything. What if they didn't? I'd love it if I could get Chrome or Firefox to cease rendering on an error when testing my own pages. Never would be able to turn it on to browse the net at large because most pages have invalid markup, but it would be nice if browsers would at least help you learn standards instead of helpfully rendering everything thrown at them.