What is the best practice to pre-select an <option> in a <select>?
According to different websites, both works. But which one is better? the most compatible?
<option selected="selected">Foo</option>
<option selected>Bar</option>
If you are writing XHTML, selected="selected" is required.
If you are writing HTML, selected is fewer keystrokes and fewer bytes.
Neither has better compatibility with browsers.
According to the HTML4 standard, using selected implies it is selected="selected". So the standard already defines both are supported. Their code sample already includes a code block just using selected.
For compatibility with XHTML (specifically XML is the 'problem' here), there is a selected="selected" option.
HTML5 also defines selected on its own is valid, so no need to worry.
来源：https://stackoverflow.com/questions/31782817/best-practice-for-html-option-selected-vs-selected-selected