app.css 应该从 /client 目录提供.将href改成如下

app.css should be served from /client dir. Change href as below

href="/client/app.css">

在您的情况下，浏览器无法找到 app.css 并且浏览器可能会收到如下类似的响应

In your case browser is not able to found the app.css and browser might be receiving a similar response as below

这显然不是 CSS 文件的有效响应和 Mime 类型.这就是您收到上述错误的原因.

Which is clearly not a valid response and Mime-type for CSS file. That's why you are getting the above error.

正如您定义的静态内容如下

As you have defined static content to be served as below

app.use("/client", express.static(__dirname + "/client"));

将 href /app.css 更改为 /client/app.css 将解决您的问题，前提是该目录中存在 app.css.

Changing href /app.css to /client/app.css will solve your issue provided app.css is present in that directory.