The difference between code behind and front side html

Hi,
This is a basic understanding I miss, 
On our aspx page most actions can be done from code behind or from aspx.cs page 
If it is done from aspx.cs page then there is no access to the code , unlike Javascript, css or plain html, for example changing font color can be done both ways:
In code behind:  lblMessage.ForeColor = Drawing.Color.DarkRed
Or in html: style="font-family:verdana;color:DarkRed"
when should I be using code behind and when html?
If the only difference is privacy then why not using only code behind?


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