原理

通过设置虚拟主机名映射到文件目录

webView.CoreWebView2.SetVirtualHostNameToFolderMapping("demo",
         "C:/Users/ivesBao/Desktop/map/final/", CoreWebView2HostResourceAccessKind.Allow);
            webView.CoreWebView2.Navigate("http://demo/downloadmap.html");

代码

wpf代码

<Window x:Class="WebViewTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WebViewTest" xmlns:wpf="clr-namespace:Microsoft.Web.WebView2.Wpf;assembly=Microsoft.Web.WebView2.Wpf"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <wpf:WebView2 Name="webView"   Initialized="WebViewInit"/>
    </Grid>
</Window>

后台代码

using Microsoft.Web.WebView2.Core;
using System;
using System.Windows;

namespace WebViewTest
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private async void WebViewInit(object sender, EventArgs e)
        {
            //跨域
            var path = AppDomain.CurrentDomain.BaseDirectory + "WebViewCache";
            //var devSession = AppDomain.CurrentDomain.BaseDirectory + "ChromeDevSession";
            var env = await CoreWebView2Environment.CreateAsync(userDataFolder: path,
                  browserExecutableFolder: null
                  //,
                  //options: new CoreWebView2EnvironmentOptions("-disable-web-security --user-data-dir=" + devSession)
                  );
            await webView.EnsureCoreWebView2Async(env);


            webView.CoreWebView2.SetVirtualHostNameToFolderMapping("demo",
         "C:/Users/ivesBao/Desktop/map/final/", CoreWebView2HostResourceAccessKind.Allow);
            webView.CoreWebView2.Navigate("http://demo/downloadmap.html");
        }
    }
}

[参考]
使用虚拟主机名映射加载本地内容的 API

留待后查，同时方便他人