inlineCriticalCss + extractCss have problems with global styles that can cause layout shifts

[akaufmann]

🐞 Bug report

What modules are related to this issue?

• aspnetcore-engine
• builders
• common
• [x ] express-engine
• hapi-engine

Is this a regression?

No

Description

I tested the new experimental inlineCriticalCss and there are some issues with global styles defined under "styles": [...] in the angular.json file...

build": {
  ...
  "options": {
    ...
    "styles": [ ... ]
  },
  ...
}

...for example the /src/styles.[s]css or NG Material themes. To make global styles work with inlineCriticalCss you must use extractCss: true to inline some of these styles otherwise they will be added at runtime. However, this causes some selector errors in the console. So I measured the performance of different solutions with Lighthouse (Chrome 87).

inlineCriticalCss:true + extractCss:true

With this combination you get styles errors in the console. For example for NG Material.

inlineCriticalCss:true + extractCss:false

inlineCriticalCss:false + extractCss:true

PS. Ignore the problems that affect the run of Lighthouse. It was audited in an incognito window but Firebase is used and writes to IndexedDB.

tl;dr

In the current form of inlineCriticalCss it looks like you get the best performance (because of lower LCP & CLS) with global styles that can cause layout shifts without inlineCriticalCss and extractCss set to true.

🔬 Minimal Reproduction

1. Add global styles that can cause layout shifts such as .foo { display: flex; } when applied later and/or a NG Material theme to the styles option in your angular.json file.
2. Enable or disable inlineCriticalCss and extractCss in the appropriate files.

// server.ts
  server.engine(
    "html",
    ngExpressEngine({
      bootstrap: AppServerModule,
      inlineCriticalCss: true,
    })
  );

// angular.json
      "architect": {
        "build": {
          ...
          "options": {
            ...
            "styles": [
              "node_modules/@angular/material/prebuilt-themes/indigo-pink.css",
              "src/styles.scss"
            ],
            ...
          },
          "configurations": {
            "production": {
              "fileReplacements": [
                {
                  "replace": "src/environments/environment.ts",
                  "with": "src/environments/environment.prod.ts"
                }
              ],
              "optimization": true,
              "outputHashing": "all",
              "sourceMap": false,
              "namedChunks": true,
              "extractLicenses": true,
              "vendorChunk": false,
              "commonChunk": false,
              "extractCss": true,
              "aot": true,
              "buildOptimizer": true
            }
          }

3. Run NODE_ENV=production ng build --prod && ng run <projectName>:server:production.
4. Run node dist/server/main.js.

🌍 Your Environment

Angular CLI: 11.1.0-next.2
Node: 12.18.0
OS: darwin x64

Angular: 11.1.0-next.2
... animations, cli, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... platform-server, router
Ivy Workspace: Yes

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1100.4
@angular-devkit/build-angular   0.1101.0-next.2
@angular-devkit/core            11.1.0-next.2
@angular-devkit/schematics      11.1.0-next.2
@angular/cdk                    11.0.1
@angular/fire                   6.1.4
@angular/material               11.0.1
@nguniversal/builders           11.1.0-next.0
@nguniversal/express-engine     11.1.0-next.0
@schematics/angular             11.1.0-next.2
@schematics/update              0.1101.0-next.2
rxjs                            6.6.3
typescript                      4.1.3

[alan-agius4]

Hi @akaufmann, thanks for trying this new feature and the provided feedback.

Let me start by explaining the difference between extractCss and inlineCriticalCss,.

extractCss

This option enables extraction of CSS from .js into a .css file. This option is currently the default and in future it will be removed hence there wouldn't be an option to disable it. Previously, this option was used for a better DX.

inlineCriticalCss

Critical CSS is extracted and inlined into the HTML during rendering. This should improve FCP and reduce render blocking requests.

I tired to replicate the the above, using the steps you provided.

npx @angular/cli@next new universal-css
cd universal-css
ng add @angular/material@next
ng add @nguniversal/express-engine@next
in styles.scss added the mentioned CSS content
in app.component.html I referenced `.foo`
yarn build:ssr && yarn serve:ssr

The results are the below, I am not going to test using extractCss: false, because this is not recommended for production and as mentioned this is deprecated.

Without inlineCriticalCss

With inlineCriticalCss

I'll try to continue looking at this in the coming days, however, it would be great if you can share your project (even privately), or minimal reproduction as a Github project.

Thanks.

[akaufmann]

Thanks @alan-agius4 for the quick reply. I adjusted the headline for each test in my first comment to make it clearer what was set to true and what was not.

This option is currently the default and in future it will be removed hence there wouldn't be an option to disable it.

The extractCss is enabled in my app, I just wanted to check what the difference is, but thanks for clarifying that it should be true or can be omitted because this is the default for prod builds.

I see higher CLS with inlineCriticalCss: true and extractCss: true. I can't share the logic of the app but I've created a stripped down version here https://github.com/akaufmann/inlineCriticalCss-issue1918 with some lower numbers than the real app.

Here are the numbers I see with both set to true:

When I set inlineCriticalCss: false I see these numbers (side note: LCP is inconsistent from run to run).

[alan-agius4]

Thanks for the repro @akaufmann, I will look at it in next couple of days.

[alan-agius4]

Hi @akaufmann,

I did take a look at the reproduction and found that;

• Critters is treating the CSS class .md\:flex as a pseudo-classes instead of a CSS selector, this is causing the class not to be inlined and caused a drastic CLS. I have PR for this fix: handle pseudo-class in middle of selector GoogleChromeLabs/critters#68
• LCP, I download the reproduction with and without inlineCriticalCss, I get a constant LCP of ~1.2ms/1.4ms

[akaufmann]

Hi @alan-agius4!

LCP, I download the reproduction with and without inlineCriticalCss, I get a constant LCP of ~1.2ms/1.4ms

Yes correct, I see that too. In both cases, the numbers for LCP fluctuate in the same range for multiple runs.

Thanks @alan-agius4 for finding time to look at this so quickly and creating a fix in the Critters repo.

[alan-agius4]

The reason why LCP is fluctuating is because the duration of fetching external font resources varies between runs.

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