On Windows 7 (or Vista) I use
New Tools: Website Speed Test and HTML Validator (2016/05/16 17:13)
We have added two new tools to our online toolbox – Online Domain Tools. The first one is called Website Speed Test. This tool deeply analyzes your web site in order to reveal how fast are the web pages delivered and rendered to the end user. There are tens of different aspects that are included in the analysis, including counting the number of needed TCP connections and DNS lookups, checking that CSS and JS dependencies are used and loaded properly and are not too heavy, checking if your web server uses caching well, analyzing the critical rendering path, testing if the number of DOM elements is reasonable, checking if minification and compression algorithms are applied, etc. The page load speed is a relevant factor for today's search engines. The faster the page loads across different platforms, the better for the web page ranking. This is also one of the results of the analysis of Google search result page ranking.
The second tool is also intended for web developers. It is HTML5 Validator. This tool was missing in our toolbox for very long time, so we are very happy we were able to add it. It is based on W3C (the consortium that created the HTML5 standard) HTML validation engine, so the validation results are the best possible. Many, if not most, of the web pages on the Internet are not valid according to the HTML5 standard. Non-valid pages can render differently in different browsers or on different devices, because once the code violates the HTML standard, the expected behavior is simply undefined. In extreme cases, this can make the web pages dysfunctional or unusable for some visitors.