Where I can find the specs for the default user agent styles values?
You can find a fairly recent copy of WebKit's UA stylesheet here.
And here are the relevant rules for the ::-webkit-search-decoration
pseudo-element:
input[type="search"]::-webkit-search-decoration {
-webkit-appearance: searchfield-decoration;
display: block;
-webkit-flex: none;
-webkit-align-self: flex-start;
margin: auto 0;
}