I completely agree. The people advocating for this type of development style are in another universe. Web is incredibly fragile.
There's a vast difference between a page being degraded by all browsers in a consistent manner per W3C specs (especially the critical parts of a webpage such as JS execution or malformed HTML) vs the damn thing breaking in such a unique way that the web devs will never be able to fix the page for this new browser while getting it to work the same for others. Worst case would be security is compromised and that is a very long list of things to implement in both the HTTP layer and browser behavior before you even get started trying to render a page.
Web devs shouldn't be fixing pages for individual browsers but instead using features conservatively and degrading gracefully where features are not available. Browser bugs shuld be fixed in the browser.
And for 99% of websites security doesn't matter at all because it's just a one-off visit to read an article or look at some funny pictures without any user account.
There's a vast difference between a page being degraded by all browsers in a consistent manner per W3C specs (especially the critical parts of a webpage such as JS execution or malformed HTML) vs the damn thing breaking in such a unique way that the web devs will never be able to fix the page for this new browser while getting it to work the same for others. Worst case would be security is compromised and that is a very long list of things to implement in both the HTTP layer and browser behavior before you even get started trying to render a page.