<p>H<sub>2</sub>O</p>

H~2~O

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document SYSTEM "CommonMark.dtd">
<document xmlns="http://commonmark.org/xml/1.0">
  <paragraph>
    <text xml:space="preserve">H</text>
    <subscript>
      <text xml:space="preserve">2</text>
    </subscript>
    <text xml:space="preserve">O</text>
  </paragraph>
</document>

<p>f<sub>i</sub> = f<sub>i - 2</sub> + f<sub>i - 1</sub>.</p>

f~i~ = f~i - 2~ + f~i - 1~.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document SYSTEM "CommonMark.dtd">
<document xmlns="http://commonmark.org/xml/1.0">
  <paragraph>
    <text xml:space="preserve">f</text>
    <subscript>
      <text xml:space="preserve">i</text>
    </subscript>
    <text xml:space="preserve"> = f</text>
    <subscript>
      <text xml:space="preserve">i - 2</text>
    </subscript>
    <text xml:space="preserve"> + f</text>
    <subscript>
      <text xml:space="preserve">i - 1</text>
    </subscript>
    <text xml:space="preserve">.</text>
  </paragraph>
</document>

<p><del>H<sub>2</sub>O</del></p>

~~H~2~O~~

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document SYSTEM "CommonMark.dtd">
<document xmlns="http://commonmark.org/xml/1.0">
  <paragraph>
    <strikethrough>
      <text xml:space="preserve">H</text>
      <subscript>
        <text xml:space="preserve">2</text>
      </subscript>
      <text xml:space="preserve">O</text>
    </strikethrough>
  </paragraph>
</document>

<p>~~H<sub>2</sub>O~~</p>

~~H~2~O~~

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document SYSTEM "CommonMark.dtd">
<document xmlns="http://commonmark.org/xml/1.0">
  <paragraph>
    <escaped_tag~~>
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      <text xml:space="preserve">O</text>
    </escaped_tag>
  </paragraph>
</document>