Introduced additional examples for parent/child DOM traversal and event propagation, including phases like capture and bubbling. Updated index.html to reference the new `event_2.md` file, highlighting expanded content on JavaScript events.

Uncommented the Bootstrap section in the index.html file, which enables the Bootstrap section on the website. Also, added substantial content to the newly created md/html/bootstrap.md file to cover Bootstrap related materials in detail. Corrected the CSS title format in the md/html/css.md file for proper markdown rendering.

The lang attribute does not default to English. It defaults to an unknown, which is an accessibility issue.