As noted by @bradfitz, this doesn't seem to affect rendered .go files. There is no leading tab character inserted in HTML on pages like https://golang.org/src/net/http/server.go:

It does affect other text files without an extension like https://golang.org/LICENSE, https://golang.org/AUTHORS, and files with non-go extension like https://golang.org/src/make.bash.

The leading tabs were always there.
fmt.Fprintf(w, "<span id=\"L%d\" class=\"ln\">%6d</span>\t", line, line)
Removing \t from there fixes it. But I am wondering the \t was there for a reason in the first place ?

The CSS change which caused this effect is golang/tools@4bc20fc#diff-62ab65160863cb8b1bd55a2653402fe4, which was made to fix another tab issue.

Specifically, if I disable these 2 properties from style.css .ln { class -

* Ensure 8 characters in the document - which due to floating
* point rendering issues, might have a width of less than 1 each - are 8
* characters wide, so a tab in the 9th position indents properly. See
* https://github.com/webcompat/web-bugs/issues/17530#issuecomment-402675091
* for more information. */
display: inline-block;
width: 8ch;

the leading space goes away, although the tab is still there in the html.

/cc @bradfitz @dmitshur @andybons @kevinburke for decision.

My guess is the tab was used for adding spacing. If no evidence otherwise can be found, I think we should remove it and implement spacing with a margin-right CSS property on ln class, if more spacing is needed.

@agnivade Where in the source code is that fmt.Fprintf call? Can you confirm that it’s only there when rendering non-.go files, but not when rendering .go ones, consistent with the behavior described in my comment above? It’d be great to learn why that difference exists, if possible.

Thank you for investigating this issue, it is helpful.

Where in the source code is that fmt.Fprintf call

godoc/format.go - FormatText()

Can you confirm that it’s only three when rendering non-.go files, but not when rendering .go ones, consistent with the behavior described in my comment above?

Correct. When rendering go files formatGoSource is called which does not have a trailing tab in its fmt.Fprintf call. See godoc/server.go formatGoSource()

Let me know if removing \t is the consensus. I can send a CL. I already have it ready.

Based on the history of events, I understand the tab character was used to ensure correct spacing. It's no longer needed and actively harmful after the change in golang/tools@4bc20fc. For that reason, and also because having it there makes copying the text harder (it includes a leading tab character not present in the source file), we should remove it.

When removing it, we just need to check that various non-.go files render okay and that it doesn't cause any spacing issues. If it does, we should address those with CSS instead of a tab character. But after the change golang/tools@4bc20fc, I expect spacing to work well in all cases. /cc @kevinburke and @mvdan, who worked on that CL.

Thanks @agnivade!

Did some testing on several pages, with both Chrome and FF. Didn't see any issues. Will send the CL.

Change https://golang.org/cl/160917 mentions this issue: godoc: remove leading tabs while formatting text