html: case not sensitive
css: case not sensitive
javascript: case sensitive
But but but these three are interrelated, so when used together to produce a change ---- TagName, ClassName, Id these three things is the link they interrelated.
When we operate through the js dom when
ID document.getElementById here is case -sensitive
document.getElementByClassName ClassName here is case -sensitive
document.getElementByTagName here TagName case- not sensitive
When we set the style by css when
#myID {...} ID here is case -sensitive
.myClass {...} ClassName where the case- sensitive
SPAN {...} TagName here the case is not sensitive