WHAT IS CHARACTER ENCODING.WHY IT IS NECESSARY IN HTML-5 ?

Answer 1

Utf8 is basically like a folder in which all the characters are available no matter what language it is, its 8 bit so 2^8 available digits=256. more or less this covers all your needed items. Previously ASCII 7 was being used but, due to some limitations(I don't know, there must have been some for sure) we got an upgrade, well there are utf16 and utf32 also available with the corresponding size of 16 and 32 size;
HappyCoding

http://artsites.ucsc.edu/ems/music/equipment/computers/bits_bytes/bits_bytes.html

Answer 2

to make browser display the text content normally. ex. : tranditional chinese characters or french characters like é , ê, á, æ...
recommand use of utf-8

Answer 3

To fulfil the requirements of the current browsers.