I see it with the same Firefox on macOS 10.12.6. Chrome here also appears ok.

Adding a white-space CSS property to the pre tag has this behavior when set to pre and pre-wrap. pre-line adds an indentation back in but causes white-space problems with the line numbers.

From https://developer.mozilla.org/en-US/docs/Web/CSS/white-space:

pre
Sequences of whitespace are preserved. Lines are only broken at newline characters in the source and at <br> elements.
pre-wrap
Sequences of whitespace are preserved. Lines are broken at newline characters, at <br>, and as necessary to fill line boxes.
pre-line
Sequences of whitespace are collapsed. Lines are broken at newline characters, at <br>, and as necessary to fill line boxes.

This looks like a Firefox problem. <pre> should have white-space: pre; set by default and this page doesn't override that. No property is displayed in the inspector until added manually, but anything non-pre doesn't have this behavior.

I'm not seeing anything for this here: https://bugzilla.mozilla.org/buglist.cgi?quicksearch=white-space. If nothing different is found and somebody with a Bugzilla account doesn't do it first I'll open up a bug there this week.

This seems to be a macOS+FF problem. In my ubuntu 16.04+FF 57.0.4, it works fine.

Update - Tested with Windows 10 + FF 57.0.4. Works fine there too.

Works fine here: OpenSuse Leap 42.3 + FF 52.5.3 (64-bit).

@agnivade which FF?

@pciet - updated my comment.

https://bugzilla.mozilla.org/show_bug.cgi?id=1432198

Timed out in state WaitingForInfo. Closing.

(I am just a bot, though. Please speak up if this is a mistake or you have the requested information.)

@gopherbot the bugzilla issue has been put into a component and set to priority 3 but no other progress has been made. There's a smaller example linked there that shows the problem.

hmm, gopherbot doesn't seem happy with me trying to reopen this. I want to take a crack at this, it's pretty annoying and I probably wrote the tab-indenting code in the first place so a fix is on me.

So this is weird:

https://jsbin.com/cavayixako/edit?html,output

I can only reproduce it with Menlo, another font does not show the same issue.

With the Menlo font in Firefox, a tab character starting in column 9 has zero width.

I can't reproduce that JSBin in Chrome.

It's a hack but you can work around it by adding another   to start the tab character in column 10.

Change https://golang.org/cl/131716 mentions this issue: godoc: fix indenting on Firefox