You can't, since only a subset of HTML tags are supported in the network's visual editor.
The following is taken directly from the Help Center on Formatting:
When posting on TeX - LaTeX Stack Exchange, you can use the visual editor (for questions and answers, but not comments), Markdown, and some HTML. If you're new to Markdown, you may want to visit the official Markdown syntax reference page.
"Some HTML" here refers to the following tags:
<a> - hyperlink.
<b> - bold, use as last resort <h1>-<h3>, <em>, and <strong> are
<blockquote> - specifies a section that is quoted from another source.
<code> - defines a piece of computer code.
<del> - delete, used to indicate modifications.
<dd> - describes the item in a <dl> description list.
<dl> - description list.
<dt> - title of an item in a <dl> description list.
<em> - emphasized.
<h1>, <h2>, <h3> - headings.
<i> - italic.
<img> - specifies an image tag.
<kbd> - represents user input (usually keyboard input).
<li> - list item in an ordered list <ol> or an unordered list <ul>.
<ol> - ordered list.
<p> - paragraph.
<pre> - pre-element displayed in a fixed width font and and
unchanged line breaks.
<s> - strikethrough.
<sup> - superscript text appears 1/2 character above the baseline
used for footnotes and other formatting.
<sub> - subscript appears 1/2 character below the baseline.
<strong> - defines important text.
<strike> - strikethrough is deprecated, use <del> instead.
<ul> - unordered list.
<br> - line break.
<hr> - defines a thematic change in the content, usually via a
[some content clipped]
HTML tags unlisted above are stripped from the output. They may render in the client preview, but they will always be removed on the server.
You must enter the tags exactly as shown. Any deviation from this list—adding extra spaces, using single quote or no quotes, etc.—means the tag will be stripped.
We do not (and will not) allow
<table> tags. Sorry. This is intentional and by design. If you need a quick and dirty "table", use
<pre> and ASCII layout.