The start tag for <tbody> is optional. That is, you can leave it out, but it is automatically inserted by the browser whenever needed.
And it is needed, because the rules say you can't have a tr directly in a table. The only children of a table element can be caption, colgroup, thead, tbody and tfoot.
So, if the browser encounters a tr outside a tbody, it inserts a tbody which the tr will reside in.
For a more formal way of saying this, see http://www.w3.org/TR/html-markup/tbody.html
By the way, the very same happens with other start tags, like <body>. If you omit that, writin, say, a <h1> directly after the </head>, the browser will insert an implied <body> automatically.
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