Security Concerns

[torgo]

From #8

• Good point about progressive enhancement. We could add that as a best-practice suggestion to 4.5 "Consider the slowest, least capable.."
• Note that the "security concerns" are due to exploit code uncovered in polyfills that can't be easily/quickly patched. The point about "generally not written by browser vendors" may be a distraction that can be dropped. Cut to "may not have the infrastructure..."

[triblondon]

Reworded the 'browser vendors' point in https://github.com/w3ctag/polyfills/commit/7769b52aae97d9477a293c9f56db1a31d47afdf7