Is fallback_rewrite_css_urls enabled by default?

Yes. fallback_rewrite_css_urls is in CoreFilters, the default RewriteLevel, so it runs unless you turn it off with DisableFilters.

How do I turn off fallback_rewrite_css_urls?

Add ModPagespeedDisableFilters fallback_rewrite_css_urls on Apache or pagespeed DisableFilters fallback_rewrite_css_urls; on nginx. On IIS, add pagespeed DisableFilters fallback_rewrite_css_urls to pagespeed.config.

filter · CSS filters

Fallback CSS URL rewriting (fallback_rewrite_css_urls)

Category:: CSS
CoreFilters:: Yes
OptimizeForBandwidth:: No
Risk:: Generally safe

Rewrites resource URLs embedded in CSS files even when CSS parsing fails. Acts as a safety net for non-standard CSS that the full parser cannot handle.

When to use it

In CoreFilters, the default RewriteLevel: it runs unless you disable it.
Risk rating on these docs: Generally safe.

Risks

The CSS filters page lists no specific risks for this filter. If a page misbehaves with it, DisableFilters fallback_rewrite_css_urls turns it off for the scope you set it in.

Configuration

Enable it in the module configuration, at server, virtual-host or location scope:

Apache

ModPagespeedEnableFilters fallback_rewrite_css_urls

nginx

pagespeed EnableFilters fallback_rewrite_css_urls;

IIS (pagespeed.config)

pagespeed EnableFilters fallback_rewrite_css_urls

It is on by default under CoreFilters; to turn it off:

ModPagespeedDisableFilters fallback_rewrite_css_urls   # Apache
pagespeed DisableFilters fallback_rewrite_css_urls;   # nginx
pagespeed DisableFilters fallback_rewrite_css_urls   # IIS

Scoping, ForbidFilters and the thresholds filters read: Choosing filters .

Live example

Rewrites URLs inside CSS even when the stylesheet cannot be fully parsed.

This filter changes how the page is structured or delivered, not its size, so the example shows a source diff rather than a byte or request reduction.

See the before and after, with the source diff →

Frequently asked questions

What does the fallback_rewrite_css_urls filter do?: Rewrites resource URLs embedded in CSS files even when CSS parsing fails. Acts as a safety net for non-standard CSS that the full parser cannot handle.
Is fallback_rewrite_css_urls enabled by default?: Yes. fallback_rewrite_css_urls is in CoreFilters, the default RewriteLevel, so it runs unless you turn it off with DisableFilters.
How do I turn off fallback_rewrite_css_urls?: Add ModPagespeedDisableFilters fallback_rewrite_css_urls on Apache or pagespeed DisableFilters fallback_rewrite_css_urls; on nginx. On IIS, add pagespeed DisableFilters fallback_rewrite_css_urls to pagespeed.config.

combine_css — Combine CSS
compute_critical_css — Compute critical CSS
extend_cache_css — Extend cache for CSS
flatten_css_imports — Flatten CSS @imports
inline_css — Inline CSS
inline_google_font_css — Inline Google Fonts CSS
inline_import_to_link — Convert @import to link
move_css_above_scripts — Move CSS above scripts

This page is drawn from the fallback_rewrite_css_urls entry on CSS filters. Every filter in one table: PageSpeed filters.