Subscript and Superscript:
A Subscript is a way to display a character or a number below the normal line of type. For example: The scientiftc notation for water is H₂O. It should be written as H₂O. Here, 2 is appearing below the normal line. This is called subscript.
A Superscript is also a way to show a character or a number above the normal line of type. For example: The familiar algebra equation "a plus b the whole square" should be written as (a+b)². Here, the square value 2 is appearing above the normal line. This is called superscript.
Usually, the subscript and the superscript character or number is smaller than the rest of the text.
In HTML, the < sub > and < sup > tags are used to create subscript and superscripts respectively. The text or number given between < sub > and < /sub > will be displayed as subscript. Same as subscript, the text or number given between < sup > and < /sup > will be displayed as superscript.
HTML CODE:
< html >
< head >
< title > Subscript and Superscript
< /title >
< /head >
< body >
The scientific notation of Water is
H< sub > 2 < /sub > O < br >
(a+b) - < sup > 2 < /sup > = a< sup >2
< /sup > + 2ab + b-< sup >2 < /sup >
< /body >
< /html >

(i) Link : Make connection with a part of the content of same document or an external document.
(ii) Hyperlink : A link from a' hypertext document.
(iii) Internal Link : Connecting with another part of the same document.
(iv) Local Link : Internal Link.
(v) External Link : Connection with another hypertext document.

Attributes of < TD >, < TH > and < TR > tags:
(i) Align: Used to specify the horizontal alignment, of content within a cell. Left is the default alignment. Possible values are Right and Center.
(ii) VAlign: Used to specify the vertical alignment of the contents within a cell. Bottom is the default alignment. Possible values are Top and Middle
(iii) Width: Used to specify the width of a cell in terms of pixels or percentage.
(iv) BGcolor and Background: Bgcolor attribute is used to apply a particular colour to the background of a cell. Background attribute is used to apply an image or picture as the background of a cell.
(v) Rowspan and Colspan : Rowspan attribute is used to merge two or more cells in a row as a single cell. Colspan attribute is used to merge to two or more cells in a column as a single cell.
An HTML code to demon trate the attribute of < td > < th > and < td > tags:
< htrnl >
< head >
< title > Attributes of td, tr and th tags
< /title >
< /head >
< body >
< table border align=center >
< Caption > Govt. Hr. Sec. School, Mullai Nagar, Thiruvallur
< tr >
< th colspan=S > Boys and Girls
Strength during 2016-17 and 2017- 18 < /th >
< /ts>
< tr align=center >
< th rowspan=2 > Class < /th >
< th rowspan=2 > Group < /th >
< th colspan=Z bgcolor=silver >
2016 - 17 < /th >
2017 - 18 < /th >
< /tr >
< tr >
< th bgcolor=yellow> Boys < /th >
< th bgcolor=pink > Girls < /th >
< th bqcolor=yellow > Boys < /th >
< th bgcolor=pink> Girls < /th >
< tts >
< tr align=center >
< th rowspan=2 > XI < /th >
< th > Science < /th >
< td > 75 < /td >
< td > 82 < /td > 
< td > 65< /td >
< td > 96 < /td >
< /tr >
< tr align=center >
< th > Commerce < /th >
< td > 125 < ztd >
< td > 147 < /td >
< td > 118 < /td >
< td > 163 < /td >
<\trl >
< tr align=center >
< th rowspan=2 > XII < /th >
< th > science < /th >
< td > 86< /td >
< td > 97 < /td >
< td > 71 < /td >
< td > 106 < /td >
< \trl >
< tr align=center>
< th > Commerce < /th >
< td > 145
< td > 186
< td > 130
< td > 198
< /trl > 
< /table >
< /body >
< /html >