As @browsermator answered putting sleep before get works.

package com.tugalsan.tst.html;

import static java.lang.System.out;
import java.nio.file.Path;
import java.time.Duration;
import org.openqa.selenium.Dimension;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.edge.EdgeDriver;
import org.openqa.selenium.edge.EdgeOptions;

public class Main {

    public static void main(String... args) throws InterruptedException {
        var urlPath = Path.of("C:\\git\\tst\\com.tugalsan.tst.html\\a.html");
        var urlStr = urlPath.toUri().toString();
        var until = Duration.ofSeconds(15);
        var scrnSize = new Dimension(640, 480);
        var output = processHTML(urlStr, until, scrnSize);
        out.println(output);
    }

    public static String processHTML(String urlStr, Duration until, Dimension scrnSize) throws InterruptedException {
        WebDriver driver = null;
        try {
            var options = new EdgeOptions();
            driver = new EdgeDriver(options);
            driver.manage().timeouts().implicitlyWait(until);
            driver.manage().timeouts().pageLoadTimeout(until);
            driver.manage().window().setSize(scrnSize);
            driver.get(urlStr);
            Thread.sleep(until);
            return driver.getPageSource();
        } finally {
            if (driver != null) {
                driver.close();
            }
            if (driver != null) {
                driver.quit();
            }
        }
    }
}