This extension worked properly for me on online and localhost pages. In the case of localhost, just need to reopen the page in a new tab after extensions is installed to see it working. It's handy to have a quick look in a popup window at different smaller resolutions alongside with your desktop page. I just wish it to have an updated devices list.
The 'Responsive Tester' plugin falls under the 'Development Tools' category and provides features such as 'Webpage Integration', 'Responsive Design Testing', and 'User Agent Emulation'
It allows developers to test their web pages against common mobile, phablet, and tablet devices by emulating the user agent and screen resolution
The plugin also enables users to view websites with multiple screen modes and choose popular screens like iPhone and Samsung for testing responsive web designs
However, it is important to note that the UI of the chrome extension may not look exactly like how it would on a real hardware device, as it is a mobile device simulator
The plugin also provides a tutorial for installation and usage