@supports-like feature on <script> tag

As far as we are planning to provide low-level APIs so that libraries can
do more polyfill before a feature gets standardized, I propose that we add
an attribute to <script> tag similar to @supports rule in CSS, which
enables the UA to avoid loading modules which try to polyfill features it
supports.

However, the @supports rule provides a positive matching, while it'd be
better to have a negative matching syntax here. In addition, it probably
should not be limited to CSS features.

Any ideas?

- Xidorn