Hierarchical relationship relative path
Use the same directory: ./ or write nothing
Parent directory with: ../
Lower directory with: directory name /
Root directory :/
Absolute path to the root directory ~ / server path
Relative path ../ client route
eg: web below index.html
css (folder): index1.html, main.css
files (folder): index2.html
cited in the main.css index.html (lower directory) path css / main.css
index1.html cited main.css (same directory) path is main.css
index2.html cited in the main.css (different documents cited in the folder) path ../css/main.css