I have been delving into a book about bootstrap in order to master it. Upon downloading the bootstrap files and integrating them into my test file, test.html, I encountered an issue where the "navbar-toggle" class was not recognized by my browser and did not appear in Firebug. Despite attempting to link to the external file to access "bootstrap.min.css", verifying that the class does exist in both the local and external files, nothing seemed to function as expected. Additionally, I faced challenges with another class, such as the "navbar-brand" class being identified in Firebug but appearing differently in the "_navbar.scss" file, which seemed to materialize out of nowhere. Similarly, the "icon-bar" class was also undetected and invisible in Firebug. While inspecting Firebug, I noticed some CSS rules from the three included files (bootstrap.min.css, bootstrap-grid.min.css, bootstrap-reboot.min.css) were displayed, yet the aforementioned three classes remained elusive.
Below is a snippet from the file test.html:
<!DOCTYPE html>
<html lang="en>
...