Understanding your Lighthouse testing results
Using these results should help you create a clear action plan of what urgent fixes or updates you might need to carry out on your website. But what do these results and actions mean?
Well each category provides a report showing performance results, a breakdown of audits passed and those not applicable, plus further additional manual tests.
The Performance category results are displayed differently to the others and split up into, Metrics - provides a summary of key performance results, Diagnostics - lists audits that need attention or have failed, Passed Audits.
The other categories Accessibility, Best Practice and SEO group their audits into, Passed, Additional, Not Applicable and Failed.
The Failed audits are then further grouped under relevant areas, for example SEO - Content Best Practices, Accessibility - Contrast or Navigation.
Lighthouse also uses a colour coded flag system for each audit result that help indicate what audits have successfully passed, those that are okay, and those that need urgent attention.
- Red Flag Poor result, needs immediate attention and improvement.
- Yellow Flag Mediocre result, could do with improvement.
- Grey Flag Passes, but could be improved.
- Green Flag Passes, successfully with no further improvement needed.
These flag indicators should further help you when creating your action plan, with priority actions given to any red and yellow flagged items.
Grey areas & red herrings
Just to confuse matters slightly, not all audits it seems are scored using the full four colours, with some audits only using the grey and yellow flags! From our own experiences, research plus reading up on other peoples findings. We found that some audits only ever pass under a grey flag so will never be added to the Passed Audit list, and are classed as a fail under a yellow flag as there is need for work and improvement in this area. Though trying to find documentation as to how, what and why these audits are scored and their passes attributed has been difficult.
Another area for confusion is the "Not Applicable" results as the audits that appear here are ones that Lighthouse has just been unable to run. Now this can be because that particular audit is legitimately not relevant to your website, for example this page when tested showed under Best Practices - Detected JavaScript libraries, now we know for a fact that we don't have any JavaScript on this page so this result should be under Not Applicable.
But this is also where the red herrings may appear, because when an item or area doesn't exist and the audit can't be run, Lighthouse can't test to whether it passes or fails, resulting in it being marked as Not Applicable.
This doesn't mean that these results aren't an important indication of areas that you really need to address. Because if these are parameters that are deemed important enough to audit, then you should probably check whether they are relevant to your website or not.
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Further Resources
JasmineWS - Common lighthouse errors explained sprout of wisdom tip.
Google's Chrome for Developers documentation Lighthouse audit categories.