One can exclude a site from search results with
-site:w3schools.com. Compare searches for
inline block and
inline block -site:w3schools.com, for example.
To avoid typing this search parameter repeatedly, one can take advantage of autocomplete: put the
-site part first; then it'll come up again when you begin typing
Google offers a Chrome extension Personal blocklist precisely for this purpose. The drawback is that this solution only works in Chrome.
Custom search engine.
Google offers custom search engines which support site exclusion. A drawback is that one has to specify the site(s) to search, and . isn't allowed. A workaround is to add popular top level domains, and set the search to be internet-wide, with only a preference for the site matching these (as shown below).
Google provides a URL for this custom engine, which you can bookmark and use instead of google.com