"The frameborder attribute on the iframe element is obsolete. Use CSS instead."

As they state themselves "The frameborder attribute is not supported in HTML5. Use CSS instead."

The equivalent of frameborder in css is border: 0px;. Add it to your iframe in css.

This works in your css

iframe{
    border-width: 0px;
}

Your HTML5 markup contains an <iframe> element with a frameborder attribute in the form:

<iframe frameborder="0" … />

This attribute is no longer present in HTML5. Use CSS instead:

<iframe style="border:0;" … />

Source: Frameborder is obsolete

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