Because "looks good to me" should not be famous last words.
Check if a page loads without script or network errors.
Click on any button or a[href="#"] and expect something to happen like DOM change or XHR call.
Check if all links work (respect robots.txt) when loaded as new page. This recurses over internal pages and validates external links.
{
"^/UrlRegex": {
options: {
loadTimeout: Number,
clickTimeout: Number,
interactiveElements: Array(DOMQuerySelector...),
followInternalLinks: Boolean,
followExternalLinks: Boolean,
networkLog: String,
}
}
}