UTF-8 character with font-weight property
font-weight property doesn't work for lower than 300 value and Unicode font.
It is Helvetica and font-weight: 300; Unicode characters still bold. What can i use?
To present text in book weight (font-weight: 300 in CSS), you need a font with that weight containing all the characters you need. Apparently, you have Helvetica in book weight in your computer (the vast majority of users don’t have it, or any Helvetica) but with a limited character repertoire that lacks many letters used in Turkish. Therefore the browser picks them up from some other font, quite possibly of normal (400) weight.
If you really want book weight text, then the best option is to use a downloadable font, also known as web font. Most fonts that people have on their computers are available in normal and bold weight only, or possibly in normal weight only. Check out Google web fonts and FontSquirrel @font-face Kits to see whether they have something that suits your design. Unfortunately, free fonts that you can easily use have very often a rather limited character repertoire.