Counterpoint: Literally the only thing that allows forward compatibility on the web is permissiveness. When a browser sees something like this in CSS,

    .foo {
      someprop: boring-old-value;
      someprop: awesome-new-value;
    }

instead of old browsers that don't understand just throwing a fit and dying, they ignore stuff they don't understand and carry on. So we know that a browser which doesn't implement 'someprop: awesome-new-value' will go ahead and read 'someprop: boring-old-value' instead of stopping processing of the CSS, or worse, emitting an error and refusing to render the page altogether. Without this, it would effectively be impossible to ship new web features until old browsers had died off to <epsilon-of-users-we-don't-care-about>.