Align <hr> to the left in an HTML5-compliant way

I don't know what browsers were used for some above answers, but I found neither text-align:left nor margin-left:0 worked in both IE (11, Standards mode, HTML5) and Firefox (29).

• IE11: text-align:left works, margin-left:0 leaves rule centred.

• FF: margin-left:0 works, text-align:left leaves rule centred.

• So, either use both, or I found that margin-right:100% works for both!

One option would be to set the left margin to zero:

hr{max-width: 800px; margin-left:0;}

You're trying to use something in a way that (as Eliezer Bernart mentions.. and apparently that comment with the link to the MDN doc disappeared) no longer "works that way". You can, as long as you don't mind it being screwy in IE, just set the margin to zero - http://jsfiddle.net/s52wb/

hr {
    max-width: 100px;
    margin: 0px;
}

A better idea though would be to mimic the HR's old way of doing things by way of CSS without the use of the HR. Check out http://jsfiddle.net/p5ax9/1/ for a demo:

p:first-child:before {
    display: none;
}
p:before {
    content: " ";
    border: 1px solid black;
    margin-top: 15px;
    margin-bottom: 15px;
    display: block;
    max-width: 100px;
}