To get italic text in a tspan or text element, set the font-style to "italic" ( and back to "normal" when you're done). The "oblique" value is also available, but as far as I know, this is a synonym for italic.
<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg viewBox = "0 0 500 300" version = "1.1">
<desc>
Text example
</desc>
<g fill = "navy">
<text x = "10" y = "25" font-size = "20">
<tspan x = "10" y = "20" font-style = "italic">
Italic font-style.
</tspan>
<tspan x = "10" y = "80" font-style = "normal">
Normal (not italic and not oblique) font-style.
</tspan>
<tspan x = "10" y = "140" font-style = "oblique">
Oblique font-style.
</tspan>
</text>
</g>
</svg>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg viewBox = "0 0 500 300" version = "1.1">
<desc>
Text example
</desc>
<g fill = "navy">
<text x = "10" y = "25" font-size = "20">
<tspan x = "10" y = "20" font-style = "italic">
Italic font-style.
</tspan>
<tspan x = "10" y = "80" font-style = "normal">
Normal (not italic and not oblique) font-style.
</tspan>
<tspan x = "10" y = "140" font-style = "oblique">
Oblique font-style.
</tspan>
</text>
</g>
</svg>
Superscript and subscript are written by using the 'baseline-shift' attribute. Just set baseline-shift="super" or baseline-shift="sub" respectively.
<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg viewBox = "0 0 500 300" version = "1.1">
<desc>
Text example
</desc>
<g fill = "navy">
<text x = "10" y = "25" font-size = "20">
<tspan>
e = mc
<tspan baseline-shift = "super">
2
</tspan>
</tspan>
<tspan x = "10" y = "60">
T
<tspan baseline-shift = "sub">
i+2
</tspan>
=T
<tspan baseline-shift = "sub">
i
</tspan>
+ T
<tspan baseline-shift = "sub">
i+1
</tspan>
</tspan>
</text>
</g>
</svg>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg viewBox = "0 0 500 300" version = "1.1">
<desc>
Text example
</desc>
<g fill = "navy">
<text x = "10" y = "25" font-size = "20">
<tspan>
e = mc
<tspan baseline-shift = "super">
2
</tspan>
</tspan>
<tspan x = "10" y = "60">
T
<tspan baseline-shift = "sub">
i+2
</tspan>
=T
<tspan baseline-shift = "sub">
i
</tspan>
+ T
<tspan baseline-shift = "sub">
i+1
</tspan>
</tspan>
</text>
</g>
</svg>